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/lib/dropTarget.test.ts

@@ -0,0 +1,76 @@
+import { describe, expect, it, beforeEach, afterEach } from "vitest";
+import { dropIsBlocked, isDropTarget } from "./dropTarget";
+
+function pane(rect: Partial<DOMRect>): HTMLElement {
+  const el = document.createElement("div");
+  document.body.appendChild(el);
+  el.getBoundingClientRect = () =>
+    ({
+      left: 0,
+      top: 0,
+      right: 100,
+      bottom: 100,
+      width: 100,
+      height: 100,
+      x: 0,
+      y: 0,
+      toJSON: () => ({}),
+      ...rect,
+    }) as DOMRect;
+  return el;
+}
+
+describe("dropTarget", () => {
+  beforeEach(() => {
+    document.body.innerHTML = "";
+  });
+  afterEach(() => {
+    document.body.innerHTML = "";
+  });
+
+  it("accepts a point inside the pane", () => {
+    expect(isDropTarget(pane({}), { x: 50, y: 50 }, { devicePixelRatio: 1 })).toBe(true);
+  });
+
+  it("rejects a point outside the pane", () => {
+    expect(isDropTarget(pane({}), { x: 400, y: 50 }, { devicePixelRatio: 1 })).toBe(false);
+  });
+
+  it("converts physical pixels to CSS pixels", () => {
+    const el = pane({});
+    expect(isDropTarget(el, { x: 150, y: 150 }, { devicePixelRatio: 2 })).toBe(true);
+    expect(isDropTarget(el, { x: 150, y: 150 }, { devicePixelRatio: 1 })).toBe(false);
+  });
+
+  it("rejects a hidden pane, which has a zero-size rect", () => {
+    const el = pane({ right: 0, bottom: 0, width: 0, height: 0 });
+    expect(isDropTarget(el, { x: 0, y: 0 }, { devicePixelRatio: 1 })).toBe(false);
+  });
+
+  it("rejects every drop while a modal is open", () => {
+    const el = pane({});
+    const dialog = document.createElement("div");
+    dialog.setAttribute("role", "dialog");
+    dialog.setAttribute("aria-modal", "true");
+    document.body.appendChild(dialog);
+
+    expect(dropIsBlocked()).toBe(true);
+    expect(isDropTarget(el, { x: 50, y: 50 }, { devicePixelRatio: 1 })).toBe(false);
+
+    dialog.remove();
+    expect(dropIsBlocked()).toBe(false);
+    expect(isDropTarget(el, { x: 50, y: 50 }, { devicePixelRatio: 1 })).toBe(true);
+  });
+
+  it("rejects every drop while a blocking overlay is up", () => {
+    const el = pane({});
+    const overlay = document.createElement("div");
+    overlay.setAttribute("data-blocks-drop", "true");
+    document.body.appendChild(overlay);
+    expect(isDropTarget(el, { x: 50, y: 50 }, { devicePixelRatio: 1 })).toBe(false);
+  });
+
+  it("rejects a null pane", () => {
+    expect(isDropTarget(null, { x: 1, y: 1 }, { devicePixelRatio: 1 })).toBe(false);
+  });
+});