Fix HIGH and MEDIUM frontend defects

Files pane
- F16: a drag-out released back inside the app no longer re-imports its own
  staged copy over the container original. An in-flight flag (cleared from the
  drag plugin's `onEvent` channel, with a watchdog) suppresses the drop and the
  "Drop files into …" hint, and an exact staged-path filter is the second line
  of defence — the `path|size|modified` cache could otherwise write a
  minutes-old snapshot over a file an agent had since rewritten.
- F17: a slow upload/rename no longer yanks the user back to the directory the
  operation started in. Every operation captures its target path and re-lists
  only if the user is still there; failures go to the toast host either way.
- The grid keeps keyboard focus. Roving tabindex (one tab stop, not one per
  row) plus focus restore after navigation, rename commit/cancel and Escape.
- Transient failures now surface in `ToastHost` (z-[60], persistent aria-live)
  instead of a `role="alert"` 300 rows down a scroller or behind a modal
  overlay. The inline error is kept only for the listing failure.
- `navigate` is sequenced by generation; "Save to host…" sets `busy`.
- Grid a11y: column headers, a text affordance for folder vs file, a live
  region that is mounted empty and announces completion, Label-in-Name fixed.
- FileViewerModal: the blob URL is released only once its replacement exists;
  the preview is a focusable, named, scrollable region.

Native drop routing
- New `lib/dropTarget.ts`: the hit test now refuses a drop while any
  `[aria-modal="true"]` dialog or `[data-blocks-drop]` overlay is up, and
  checks z-order where the environment can answer it. Shared by FilesTab and
  TerminalView; App's shutdown overlay opts in.

Disk
- A partially failed reclaim says so in words ("… — 2 of 5 failed"), not by hue
  alone.
- The scan/reclaim race is closed: every mutation retires an in-flight scan, so
  a scan can no longer repaint a pre-reclaim report plus a clickable plan of
  objects that are gone. Scan is disabled while working; the status is a live
  region; a failed destructive action keeps its dialog open and reports there.
- The "unknown" layer count gets a screen-reader fallback; `--text-disabled`
  no longer carries live information.

Terminal / OAuth
- After the toast is dismissed, a truncated heuristic guess can no longer fill
  the slot that an exact OSC 8 or relay URL occupied — the detector remembers
  every exact URL and drops any candidate that is a strict prefix of one.
- The prompt is reachable by keyboard: Ctrl+Shift+O jumps to the default
  action, Escape dismisses, focus returns to the terminal, and auto-dismiss
  holds off while focus is inside. It deliberately does not steal focus.
- UrlToast renders through `ui/Button` and `--shadow-overlay`.

Elsewhere
- AuthBridgeRow: a pushed `auth-bridge-changed` status always outranks an older
  awaited toggle result.
- The last two ad-hoc byte formatters route through `lib/formatBytes`.

Contract for the backend agent: `upload_file_to_container` refusing to
overwrite must satisfy `isFileExistsError` in `src/lib/uploadErrors.ts` (marker
`FILE_EXISTS`) and accept an `overwrite` argument; the frontend turns that into
an `ui/Modal` Replace/Skip prompt rather than a raw error string.

Tests: 536 -> 627 passing. `npm run build` and `npx tsc --noEmit` green.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GBq2rGum6GX7xXgsas1fDc

app/src/components/terminal/UrlToast.tsx

@@ -1,5 +1,32 @@
-import type { CSSProperties, MouseEvent } from "react";
+import type { KeyboardEvent } from "react";
 import { isAnthropicSignInUrl, urlOrigin } from "../../lib/urlRelay";
+import Button from "../ui/Button";
+
+/**
+ * Marks the toast's subtree. `TerminalView` uses it to answer "is focus inside
+ * the thing I am about to unmount?", which is what decides whether dismissing
+ * has to hand focus back to the terminal.
+ */
+export const URL_TOAST_SELECTOR = '[data-testid="url-toast"]';
+
+/**
+ * The chord that jumps from the terminal into this toast.
+ *
+ * Bound in `TerminalView` on `document` in the capture phase, the same way
+ * `useKeyboardShortcuts` binds the app's other chords, because xterm would
+ * otherwise forward it to the shell. Shift is what keeps it clear of the
+ * terminal: plain Ctrl+O is readline's `operate-and-get-next`.
+ */
+export const URL_TOAST_SHORTCUT = "Ctrl+Shift+O";
+
+/**
+ * Marks the *default* action inside the toast, so the owner can put focus
+ * there without a ref threaded through `ui/Button` — which is a plain function
+ * component and not this file's to change. Which button it is depends on the
+ * URL (see the sign-in note below), so the attribute moves with the decision
+ * rather than the caller having to repeat it.
+ */
+export const URL_TOAST_PRIMARY_SELECTOR = '[data-url-toast-primary="true"]';
 
 interface Props {
   /** Already validated by `sanitizeRelayUrl` — this component never opens it. */
@@ -41,6 +68,28 @@ interface Props {
  * with no host round trip and no auth bridge, so it leads — and the host button
  * stays, because a user who has the auth bridge on, or who wants their existing
  * browser session, still needs it.
+ *
+ * ## Reachable without a mouse, and it does not take focus to manage it
+ *
+ * This toast is the only route to completing a sign-in started in a terminal,
+ * and it used to be mouse-only: xterm's helper textarea swallows Tab, so there
+ * was no way to reach these buttons at all from the keyboard.
+ *
+ * The obvious fix — focus the default action when the toast appears — was
+ * rejected on two counts. The terminal underneath is *live*: the user may be
+ * mid-command, and every keystroke after the steal would go to a button instead
+ * of the shell. Worse, the default action opens a URL chosen by the untrusted
+ * side of the sandbox, and a focused button is one stray Space or Enter away
+ * from doing it. This prompt exists precisely to make that a deliberate act.
+ *
+ * So focus stays where the user put it and the toast is reachable on demand:
+ * {@link URL_TOAST_SHORTCUT} jumps to the default action (the hint is on
+ * screen, next to the label, because a shortcut nobody is told about is not a
+ * route), Tab then moves between the actions normally — this subtree is not
+ * inside xterm — and Escape dismisses. Escape is handled *here*, on the
+ * toast's own subtree, rather than globally: Escape belongs to whatever is
+ * running in the terminal, and a document-level binding for it would break vim
+ * for everyone who never looked at this toast.
  */
 export default function UrlToast({
   url,
@@ -55,82 +104,56 @@ export default function UrlToast({
   // host button is the only action there is, so it stays primary.
   const signIn = !!onOpenInContainer && isAnthropicSignInUrl(url);
 
-  // Filled uses `--accent-emphasis`, never `--accent` — the latter is the
+  // `Button` already owns the filled/outlined variants — including the rule
+  // that filled uses `--accent-emphasis` and never `--accent`, which is the
   // foreground/link accent and fails WCAG AA behind white text.
-  const primaryStyle: CSSProperties = {
-    padding: "4px 12px",
-    fontSize: 12,
-    fontWeight: 600,
-    color: "#fff",
-    background: "var(--accent-emphasis)",
-    border: "1px solid transparent",
-    borderRadius: 4,
-    cursor: "pointer",
-    whiteSpace: "nowrap",
-    flexShrink: 0,
-  };
-  const secondaryStyle: CSSProperties = {
-    padding: "4px 10px",
-    fontSize: 12,
-    fontWeight: 600,
-    color: "var(--text-primary)",
-    background: "transparent",
-    border: "1px solid var(--border-color)",
-    borderRadius: 4,
-    cursor: "pointer",
-    whiteSpace: "nowrap",
-    flexShrink: 0,
-  };
-
-  /** Hover feedback for whichever button is currently the filled one. */
-  const hover = (primary: boolean) =>
-    primary
-      ? {
-          onMouseEnter: (e: MouseEvent<HTMLButtonElement>) =>
-            (e.currentTarget.style.background = "var(--accent-emphasis-hover)"),
-          onMouseLeave: (e: MouseEvent<HTMLButtonElement>) =>
-            (e.currentTarget.style.background = "var(--accent-emphasis)"),
-        }
-      : {
-          onMouseEnter: (e: MouseEvent<HTMLButtonElement>) =>
-            (e.currentTarget.style.background = "var(--bg-tertiary)"),
-          onMouseLeave: (e: MouseEvent<HTMLButtonElement>) =>
-            (e.currentTarget.style.background = "transparent"),
-        };
-
   const hostButton = (
-    <button
+    <Button
+      variant={signIn ? "secondary" : "primary"}
+      data-url-toast-primary={signIn ? undefined : "true"}
       onClick={onOpen}
+      className="flex-shrink-0"
       title={
         signIn
           ? "Open in your own browser instead — the callback then has to reach the container by some other route"
           : undefined
       }
-      style={signIn ? secondaryStyle : primaryStyle}
-      {...hover(!signIn)}
     >
       Open
-    </button>
+    </Button>
   );
 
   const containerButton = onOpenInContainer && (
     // A sign-in completed in the *container's* browser lands its callback on
     // the container's own loopback, which is where the tool waiting for it is
     // listening — no host round trip, no auth bridge.
-    <button
+    <Button
+      variant={signIn ? "primary" : "secondary"}
+      data-url-toast-primary={signIn ? "true" : undefined}
       onClick={onOpenInContainer}
+      className="flex-shrink-0"
       title="Open in a browser inside the container, and watch it in the Browser tab"
-      style={signIn ? primaryStyle : secondaryStyle}
-      {...hover(signIn)}
     >
       In container
-    </button>
+    </Button>
   );
 
+  const onKeyDown = (e: KeyboardEvent<HTMLDivElement>) => {
+    if (e.key !== "Escape") return;
+    // Scoped to this subtree, so the terminal's own Escape is untouched.
+    e.preventDefault();
+    e.stopPropagation();
+    onDismiss();
+  };
+
   return (
     <div
       className="animate-slide-down"
+      data-testid="url-toast"
       role="status"
+      aria-atomic="true"
+      aria-keyshortcuts="Control+Shift+O"
+      onKeyDown={onKeyDown}
       style={{
         position: "absolute",
         top: 12,
@@ -144,7 +167,7 @@ export default function UrlToast({
         background: "var(--bg-secondary)",
         border: "1px solid var(--border-color)",
         borderRadius: 8,
-        boxShadow: "0 4px 12px rgba(0,0,0,0.4)",
+        boxShadow: "var(--shadow-overlay)",
         maxWidth: "min(90%, 600px)",
       }}
     >
@@ -157,6 +180,11 @@ export default function UrlToast({
           }}
         >
           {label}
+          {" · "}
+          <span data-testid="url-toast-shortcut" style={{ fontFamily: "monospace" }}>
+            {URL_TOAST_SHORTCUT}
+          </span>{" "}
+          to reach the buttons, Esc to dismiss
         </div>
         <div
           data-testid="url-toast-url"
@@ -226,29 +254,15 @@ export default function UrlToast({
         </>
       )}
 
-      <button
+      <Button
+        variant="ghost"
         onClick={onDismiss}
-        style={{
-          padding: "2px 6px",
-          fontSize: 14,
-          lineHeight: 1,
-          color: "var(--text-secondary)",
-          background: "transparent",
-          border: "none",
-          borderRadius: 4,
-          cursor: "pointer",
-          flexShrink: 0,
-        }}
-        onMouseEnter={(e) =>
-          (e.currentTarget.style.color = "var(--text-primary)")
-        }
-        onMouseLeave={(e) =>
-          (e.currentTarget.style.color = "var(--text-secondary)")
-        }
+        className="flex-shrink-0"
         aria-label="Dismiss"
+        title="Dismiss (Esc)"
       >
         ✕
-      </button>
+      </Button>
     </div>
   );
 }