feat(audit): Batch 11 — UX behavior gaps (toast surface, sidebar badge)

A new in-app toast surface is the batch's spine, making the silent-failure and
global-status gaps fixable:

- Toast infra: store/toasts.ts (useToasts + toast(); auto-dismiss) + ui/Toaster.tsx
  (non-portal bottom-center stack, avoids the Fluent-portal GPU flicker). Unit-
  tested (test/toasts.test.ts). Mounted at the app root beside LiveRegion.
- UX6: shared renderer reveal.ts revealFile('open'|'folder') awaits the IPC result
  and toasts the main-process reason instead of silently no-oping on a moved/typed-
  out file. safeShowInFolder upgraded to return an error string like safeOpenPath
  (preload/mock/Api types updated).
- UX9 / UI25: a Fluent CounterBadge on the sidebar Downloads nav item shows the
  active (downloading + queued) count from any tab, via a count-only App selector
  that doesn't re-render on progress ticks.
- UX15: cancelAll store action + a "Cancel all (N)" header button.
- UX24: the disabled "Check watched" button uses disabledFocusable + a Hint that
  explains it needs a watched source.
- UX12: the Terminal gate gains an inline "Enable custom commands" button, so it's
  no longer a dead-end pointing at another screen.
- L144: the DownloadBar duplicate guard now also checks history — a URL downloaded
  earlier warns "Already downloaded: …" (dupInHistory) vs "Already in your queue: …".

Deferred with rationale (CODE-AUDIT.md): UX7/UX8 (Settings IA redesign — visual),
UX18/UX26 (subjective/visual polish), L131 (arguably by-design, needs live taskbar),
L142 (history/templates/sources IPC-return reconciliation — a focused own PR).

Verified: typecheck (node+web) + 279 tests + eslint + production build green;
touched files prettier-clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/preload/index.ts

@@ -80,7 +80,8 @@ const api = {
 
   openUrl: (url: string): Promise<void> => ipcRenderer.invoke(IpcChannels.openUrl, url),
 
-  showInFolder: (path: string): Promise<void> => ipcRenderer.invoke(IpcChannels.showInFolder, path),
+  showInFolder: (path: string): Promise<string> =>
+    ipcRenderer.invoke(IpcChannels.showInFolder, path),
 
   readClipboard: (): Promise<string> => ipcRenderer.invoke(IpcChannels.clipboardRead),