feat(admin): phase 4 — Cmd+K command palette

Global search modal: actions + users + tests + classes.

- GET /api/admin/search?q=X (~50L controller): 3 parameterized LIKE queries, admin-only

- frontend/js/admin/palette.js (~366L): custom lightweight modal (not LS.modal — footer-button oriented), Ctrl+K/Cmd+K capture-phase override of generic /js/search.js, debounce 150ms, race-guard via _reqSeq, min-query 2 chars, 8 hardcoded actions, ↑↓ wrap + Enter, click-outside close

- adminGlobalSearch helper: drop ignored 'limit' param (server hardcodes 5/3/3)

window.AdminPalette = { open, close, isOpen } exposed for Phase 5/6 use.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/admin.html

@@ -2006,6 +2006,7 @@
   <script src="/js/admin/sections/questions.js"></script>
   <script src="/js/admin/sections/users.js"></script>
   <script src="/js/admin/sections/sessions.js"></script>
+  <script src="/js/admin/palette.js"></script>
   <script src="/js/admin/admin.js"></script>
   </div>
 </div>