feat(phase-4): frontend data layer — API client (13 new functions), folders store, documents store extensions, routes

- Extended listDocuments to accept folderId, q, sort, order query params
- Added 6 folder API functions: listFolders, createFolder, getFolder, renameFolder, deleteFolder, moveDocument
- Added 4 share API functions: createShare, listShares, deleteShare, getSharedWithMe
- Added 2 preference API functions: getMyPreferences, updateMyPreferences
- Added getDocumentContentUrl helper (returns URL string, no fetch)
- Created useFoldersStore with full CRUD, navigation state, and breadcrumb support
- Extended useDocumentsStore with currentFolderId, searchQuery, sortField, sortOrder refs
- Added debounced searchQuery watcher (300ms, 2-char minimum, T-04-08-03)
- Added shareDocument, revokeShare, listShares actions to documents store
- Added /folders/:folderId and /shared routes with requiresAuth guard

frontend/src/router/index.js

@@ -38,6 +38,20 @@ const routes = [
   // Phase 2 — authenticated routes
   { path: '/account', component: () => import('../views/AccountView.vue') },
   { path: '/admin', component: () => import('../views/AdminView.vue') },
+
+  // Phase 4 — folder and sharing routes
+  {
+    path: '/folders/:folderId',
+    name: 'folder',
+    component: () => import('../views/FolderView.vue'),
+    meta: { requiresAuth: true },
+  },
+  {
+    path: '/shared',
+    name: 'shared',
+    component: () => import('../views/SharedView.vue'),
+    meta: { requiresAuth: true },
+  },
 ]
 
 const router = createRouter({