Phase 3: Graph View, Backlinks UI, Quick Switcher, Dark Mode, Export

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

src/types/index.ts

@@ -106,6 +106,103 @@ export interface DocumentsResponse {
   documents: Document[]
 }
 
+// Graph types
+export interface GraphNode {
+  id: string
+  title: string
+  type: 'document'
+}
+
+export interface GraphEdge {
+  source: string
+  target: string
+  type: 'reference'
+}
+
+export interface GraphStats {
+  total_documents: number
+  total_references: number
+  orphaned_documents: number
+}
+
+export interface GraphData {
+  project_id: string
+  nodes: GraphNode[]
+  edges: GraphEdge[]
+  stats: GraphStats
+}
+
+// Backlinks types
+export interface BacklinkItem {
+  document_id: string
+  title: string
+  project_id: string
+  project_name: string
+  excerpt: string
+  updated_at: string
+}
+
+export interface BacklinksResponse {
+  document_id: string
+  backlinks_count: number
+  backlinks: BacklinkItem[]
+}
+
+export interface OutgoingLinkItem {
+  document_id: string
+  title: string
+  project_id: string
+  project_name: string
+  exists: boolean
+  updated_at: string
+}
+
+export interface OutgoingLinksResponse {
+  document_id: string
+  outgoing_links_count: number
+  outgoing_links: OutgoingLinkItem[]
+}
+
+export interface DocumentLinkRef {
+  document_id: string
+  title: string
+  anchor_text: string
+}
+
+export interface DocumentLinksResponse {
+  document_id: string
+  outgoing_links: DocumentLinkRef[]
+  backlinks: DocumentLinkRef[]
+}
+
+// Search / Quick Switcher types
+export interface SearchResult {
+  id: string
+  type: 'document' | 'project'
+  title: string
+  subtitle: string | null
+  highlight: string | null
+  icon: string
+  project_id?: string
+}
+
+export interface SearchResponse {
+  query: string
+  results: SearchResult[]
+  total: number
+  search_type: string
+}
+
+// Export types
+export interface DocumentExportResponse {
+  // The API returns the raw file, not JSON
+  // This is for type safety on the response
+}
+
+export interface ProjectExportResponse {
+  // The API returns the raw file, not JSON
+}
+
 // Form inputs
 export interface LoginForm {
   username: string