feat: universal sidebar via sidebar.js + stale ID cleanup

- Add js/sidebar.js: generates full sidebar HTML into #app-sidebar,
  handles role-based visibility, active link (with prefix matching),
  toggle wiring, collapsed state, board/features/notif init
- Replace <aside class="sidebar">...</aside> with <aside id="app-sidebar">
  across all 35 standard-layout pages via scripts/apply-sidebar.js
- Add notifications.js to 5 pages that were missing it
- Fix api.js initPage(): skip toggle re-wiring if data-sb-wired set,
  fix active link selector .sb-item → .sb-link
- Remove stale sbl-*/nav-admin/btn-upload-nav getElementById calls
  that crashed after sidebar replacement (lab, classes, collection,
  crossword, hangman, knowledge-map, library, pet, profile)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/css/ls.css

@@ -444,6 +444,13 @@ body {
   overflow-x: hidden;
 }
 
+/* notif wrap — visually separate from nav items */
+#notif-wrap {
+  border-top: 1.5px solid var(--border);
+  margin-top: 6px;
+  padding-top: 6px;
+}
+
 /* notif drop anchored relative to sidebar */
 .notif-drop {
   position: fixed;