refactor: extract useAutoReset hook and fix selector stability

[randroid88]

Summary

• Extract useAutoReset hook — replaces 4 identical useState + useEffect + setTimeout patterns across components
• Fix temporal selector stability — wrap useStore(temporal) with useShallow to prevent new-object-on-every-update re-renders

Depends on: #8 (browser file ops — modifies AIPanel, ExportDialog) and #9 (undo/redo — introduces temporal subscription in App.tsx)

useAutoReset hook

const [copied, setCopied] = useAutoReset(false, 2000);
// setCopied(true) → auto-resets to false after 2s

Replaces this pattern that was duplicated in 4 places:

const [copied, setCopied] = useState(false);
useEffect(() => {
  if (!copied) return;
  const timer = setTimeout(() => setCopied(false), 2000);
  return () => clearTimeout(timer);
}, [copied]);

Components updated

Component	State replaced
TranscriptEditor	copied (2s)
ExportDialog	exportSuccess (3s), exportError (3s)
AIPanel	exportedClipIndex (3s), exportClipError (3s)

Selector stability

The temporal store subscription in App.tsx creates a new object { pastStates, futureStates } on every store update, causing unnecessary re-renders. Wrapping with useShallow performs shallow equality comparison to prevent this.

Files changed

• frontend/src/hooks/useAutoReset.ts — New: reusable auto-resetting state hook
• frontend/src/App.tsx — Add useShallow to temporal selector
• frontend/src/components/AIPanel.tsx — Use useAutoReset for clip export feedback
• frontend/src/components/ExportDialog.tsx — Use useAutoReset for export feedback
• frontend/src/components/TranscriptEditor.tsx — Use useAutoReset for copy feedback

Test plan

• Copy transcript → "Copied!" resets after 2 seconds
• Export video → "Saved!" resets after 3 seconds
• Export AI clip → "Saved!" resets after 3 seconds
• Export error → error message resets after 3 seconds
• Verify no unnecessary re-renders (React DevTools profiler)