feat(desktop): upload progress on the taskbar button (cinny-desktop #10)

Upload atoms now also report into an app-wide `uploadProgressAtom`, and
`aggregateUploadProgress` turns every in-flight upload into one taskbar
state: byte-weighted percentage while uploading, indeterminate until a
size is known, red after a failure (held 4 s, then cleared), none when
done. Cancelled uploads just disappear. `useTauriTaskbarProgress` sends it
to the native `set_taskbar_progress`, at most ~4 times a second, always
ending on the latest state.

Verified with a throttled 6 MB upload: 0% → 99% at 250 ms steps, then
cleared; with the upload request aborted: 0% → error → cleared after 4 s.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/components/TauriDesktopFeatures.tsx

@@ -1,6 +1,7 @@
 import { useTauriCallPower } from '../hooks/useTauriCallPower';
 import { useTauriJumpList } from '../hooks/useTauriJumpList';
 import { useTauriThumbbar } from '../hooks/useTauriThumbbar';
+import { useTauriTaskbarProgress } from '../hooks/useTauriTaskbarProgress';
 import { useTauriSmtc } from '../hooks/useTauriSmtc';
 import { useTauriNetwork } from '../hooks/useTauriNetwork';
 import { useTauriToastActions } from '../hooks/useTauriToastActions';
@@ -18,6 +19,7 @@ export function TauriDesktopFeatures(): null {
   useTauriCallPower(); // P5-46 no-sleep during calls
   useTauriJumpList(); // P5-36 Windows jump list of recent rooms
   useTauriThumbbar(); // P5-44 taskbar thumbnail toolbar (mute/deafen/end)
+  useTauriTaskbarProgress(); // cinny-desktop #10 upload progress on the taskbar button
   useTauriSmtc(); // P5-43 system media transport controls
   useTauriNetwork(); // P5-49 network-change awareness → sync retry
   useTauriToastActions(); // P5-41/35 rich toast click → open room, quick reply → send