feat(admin): phase 3 — dashboard #overview landing

GET /api/admin/overview returns 24h digest (~0.08ms/call).

- adminController.getOverview: 7 prepared statements (users 24h, sessions 24h, active users, classes count, failed sessions, banned this week, top-5 sessions)

- new section frontend/js/admin/sections/overview.js (~205L): bento-grid cards, alerts (only when >0), top-5 table, quick-links

- nav-item + tab-pane reordered: #overview is now default; #stats remains routable

Auth: admin-only (inside requireRole('admin') block, sibling of /stats).

Backward compat: all 13 existing routes unchanged.

Known follow-ups (post-merge polish):

- activeClasses counts all (label could be 'Всего классов')

- failedSessions24h includes in_progress (could tighten to abandoned only)

- topSessions24h drops NULL-score completed rows

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

js/api.js

@@ -150,6 +150,7 @@ async function importQuestions(formData) {
 
 /* ── admin ────────────────────────────────────────────────────────────── */
 async function adminGetStats()              { return req('GET',   '/admin/stats'); }
+async function adminGetOverview()           { return req('GET',   '/admin/overview'); }
 async function adminGetUsers(params = {})   {
   const p = new URLSearchParams();
   if (params.page)  p.set('page',  params.page);
@@ -939,7 +940,7 @@ window.LS = {
   register, login, fetchMe, updateProfile,
   getSubjects, updateSubject, getTopics,
   startSession, sendAnswer, finishSession, getResult, getHistory, getWeakTopics, getStudentStats, getSessionQuestions,
-  adminGetStats, adminGetUsers, adminUpdateRole, adminGetUserSessions, adminGetSessions, adminGetSessionDetail, adminClearUserSessions, adminUpdateUser, adminBanUser, adminDeleteUser,
+  adminGetStats, adminGetOverview, 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,