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>

js/api.js

@@ -151,6 +151,10 @@ async function importQuestions(formData) {
 /* ── admin ────────────────────────────────────────────────────────────── */
 async function adminGetStats()              { return req('GET',   '/admin/stats'); }
 async function adminGetOverview()           { return req('GET',   '/admin/overview'); }
+async function adminGlobalSearch(q) {
+  // Limits are hardcoded server-side (top 5 users / 3 tests / 3 classes).
+  return req('GET', `/admin/search?q=${encodeURIComponent(q)}`);
+}
 async function adminGetUsers(params = {})   {
   const p = new URLSearchParams();
   if (params.page)  p.set('page',  params.page);
@@ -940,7 +944,7 @@ window.LS = {
   register, login, fetchMe, updateProfile,
   getSubjects, updateSubject, getTopics,
   startSession, sendAnswer, finishSession, getResult, getHistory, getWeakTopics, getStudentStats, getSessionQuestions,
-  adminGetStats, adminGetOverview, adminGetUsers, adminUpdateRole, adminGetUserSessions, adminGetSessions, adminGetSessionDetail, adminClearUserSessions, adminUpdateUser, adminBanUser, adminDeleteUser,
+  adminGetStats, adminGetOverview, adminGlobalSearch, adminGetUsers, adminUpdateRole, adminGetUserSessions, adminGetSessions, adminGetSessionDetail, adminClearUserSessions, adminUpdateUser, adminBanUser, adminDeleteUser,
   getQuestions, createQuestion, duplicateQuestion, updateQuestion, deleteQuestion, importQuestions,
   getClasses, createClass, getClassDetail, updateClass, deleteClass, kickMember, addClassMember, createAssignment, createDirectAssignment, updateAssignment, deleteAssignment,
   regenerateInviteCode, classJournal,