refactor(frontend): extract OverviewTab safely
- New component: frontend/src/lib/components/OverviewTab.svelte
- Props: selectedNode, notes, worklog, formatDate, nodeKindLabel
- Events: createNote, addFile, createAction, switchTab, openNote
- Pure display component, no internal state
- App.svelte changes:
- Replaced inline overview markup with <OverviewTab>
- Removed overview-related CSS (overview, meta-grid, qa-btn, etc.)
- openCreateNote() call replaced with setActiveTab('notes') only
(create note is now handled by NotesTab component)
- Build: npm run build ✅, go test ./... ✅