Features:
- GraphView.vue: SVG-based force-directed graph visualization for projects
- QuickSwitcher.vue: Cmd+K modal with fuzzy search via /search API
- Dark Mode: Theme toggle in Header, persisted in localStorage, system pref support
- Backlinks UI: Incoming and outgoing links in DocumentView
- Export: Document (markdown/JSON) and Project (ZIP/JSON) export with download
- New composables: useTheme.ts for dark/light/system theme management
- New store methods: fetchBacklinks, fetchOutgoingLinks, search, exportDocument, fetchProjectGraph, exportProject
- TypeScript types for all Phase 3 API responses
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