refactor(frontend): modularise App.svelte — Phase 1-4
• Create docs/frontend-architecture.md and docs/frontend-change-map.md
• Extract API layer: lib/services/ (wails, notes, files, search, inbox,
trash, sync, journal, actions, links, activity, nodes, suggestions,
today, browserEvents)
• Extract ErrorBanner.svelte component
• Extract CaptureDropOverlay.svelte component
• Extract OverviewTab.svelte component
• Extract NotesTab.svelte component
• Wire all extracted components into App.svelte
• Build passes (npm run build ✓)