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/ui/TypedConfirmModal.test.tsx

@@ -103,6 +103,19 @@ describe("TypedConfirmModal", () => {
     expect(onConfirm).not.toHaveBeenCalled();
   });
 
+  it("carries a failed attempt inside the dialog, as an alert", () => {
+    // The caller keeps this dialog open when the deletion fails, because the
+    // panel behind it is several screens long and its error line sits at the
+    // top — nowhere near the row this was opened from.
+    renderModal({ error: "volume triple-c-home-p-whp is in use by a running container" });
+    expect(screen.getByRole("alert")).toHaveTextContent(/in use by a running container/);
+  });
+
+  it("says nothing about failure when there has been none", () => {
+    renderModal();
+    expect(screen.queryByRole("alert")).not.toBeInTheDocument();
+  });
+
   it("cannot be satisfied by an empty box when there is no name to type", () => {
     const { confirm } = renderModal({ expected: "" });
     expect(confirm).toBeDisabled();

app/src/components/ui/TypedConfirmModal.tsx

@@ -14,6 +14,13 @@ interface Props {
   onConfirm: (typed: string) => void;
   onCancel: () => void;
   busy?: boolean;
+  /**
+   * Why the last attempt did not happen. The caller keeps the dialog open when
+   * its action fails, so the failure has to be readable *here* — the panel
+   * behind this one is several screens long and its error line is at the top
+   * of it, which is not where the user is looking.
+   */
+  error?: string | null;
 }
 
 /**
@@ -44,6 +51,7 @@ export default function TypedConfirmModal({
   onConfirm,
   onCancel,
   busy = false,
+  error = null,
 }: Props) {
   const [typed, setTyped] = useState("");
   const inputRef = useRef<HTMLInputElement>(null);
@@ -104,12 +112,22 @@ export default function TypedConfirmModal({
             {matches ? (
               <span className="text-[var(--text-secondary)]">Name matches.</span>
             ) : (
-              <span className="text-[var(--text-disabled)]">
+              // Not disabled content — the gate is live and waiting on the
+              // user. `--text-disabled` is ~4.1:1 and fails AA at 12px.
+              <span className="text-[var(--text-secondary)]">
                 Waiting for the exact project name.
               </span>
             )}
           </p>
         </div>
+        {error && (
+          // Rendered last, next to the button that was just pressed, and as an
+          // `alert` so it is announced on arrival rather than waiting to be
+          // found.
+          <p role="alert" className="text-[var(--error)]">
+            {error}
+          </p>
+        )}
       </div>
     </Modal>
   );