refactor: split app.js (8875 lines) into 12 ES modules

frontend/js/ structure:
  state.js      (55 lines)  — Shared mutable state, constants
  utils.js      (510 lines) — EXT_ICONS, getFileIcon, escapeHtml, safeCreateIcons
  auth.js       (547 lines) — api(), AuthManager, initLoginForm, AdminPanel
  search.js     (1106 lines)— SearchHistory, QueryParser, Autocomplete, performSearch
  sidebar.js    (1091 lines)— Vault tree, sidebar filter, TagFilterService, loadTags
  viewer.js     (1554 lines)— openFile, Outline, ScrollSpy, Frontmatter, Editor
  ui.js         (2250 lines)— Theme, Toast, Sidebar, Dropdowns, Tabs, ContextMenu
  dashboard.js  (461 lines) — Dashboard widgets (Recent, Stats, Bookmarks)
  config.js     (999 lines) — Config panel, Hidden files, About, Sidebar tabs
  sync.js       (436 lines) — SSE/IndexUpdateManager, PWA registration
  graph.js      (401 lines) — GraphViewManager (force-directed canvas graph)
  legacy.js     (550 lines) — Remaining bridge functions (goHome, showWelcome, initSearch)
  app.js        (80 lines)  — Thin orchestrator: imports all modules, calls init()

index.html: switched from <script src="app.js"> to <script type="module" src="js/app.js">
Original app.js preserved for backward compatibility.
All 14 modules pass node --check syntax validation.

frontend/index.html

@@ -1133,6 +1133,6 @@
     <div class="find-error" id="find-error" hidden></div>
   </div>
 
-  <script src="/static/app.js"></script>
+  <script type="module" src="/static/js/app.js"></script>
  </body>
  </html>