feat(web): sync-conflict banner at top of media item page + end-time help row

- A progress sync conflict used to be flagged only inside the progress
  card, far down a long page. An unresolved conflict now also renders
  a full-width warning strip at the very top (under the app bar) that
  opens the same resolution modal on click. Rendered from the same
  ActiveConflict the progress section uses — absent when clean.
- Shortcuts dialog Mouse section gains an 'End time' row: click to
  cycle total → book left → chapter left (§10's toggle was
  undiscoverable).

web/src/audio-keys.ts

@@ -232,6 +232,10 @@ export const MOUSE_GUIDE: readonly PointerGuideRow[] = [
   { label: "Shift + click", description: "Jump to the nearest chapter line" },
   { label: "Shift + drag", description: "Precision rail (±5 min) around the nearest line" },
   { label: "Wheel", description: "Nudge ±5s (Shift: ±60s)" },
+  {
+    label: "End time",
+    description: "Click to cycle the −label: total → book left → chapter left",
+  },
 ];
 
 export const TOUCH_GUIDE: readonly PointerGuideRow[] = [