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>

plans/admin-redesign/PLAN.md

@@ -37,7 +37,7 @@
 
 - [x] Phase 1: Hash-router [domain: frontend] → [subplan](./phase-1-hash-router.md)
 - [x] Phase 2: Split admin.html → per-section modules [domain: frontend] → [subplan](./phase-2-split-sections.md)
-- [ ] Phase 3: Dashboard #overview [domain: fullstack] → [subplan](./phase-3-dashboard.md) (parallelizable with 4, 5)
+- [x] Phase 3: Dashboard #overview [domain: fullstack] → [subplan](./phase-3-dashboard.md) (parallelizable with 4, 5)
 - [ ] Phase 4: Cmd+K command palette [domain: fullstack] → [subplan](./phase-4-palette.md) (parallelizable with 3, 5)
 - [ ] Phase 5: Per-row quick actions [domain: frontend] → [subplan](./phase-5-quick-actions.md) (parallelizable with 3, 4)
 - [ ] Phase 6: Deep entity pages [domain: frontend] → [subplan](./phase-6-deep-pages.md)
@@ -49,8 +49,8 @@
 | Phase | Domain | Status | Review | Build | Committed |
 |-------|--------|--------|--------|-------|-----------|
 | Phase 1: Hash-router | frontend | ✅ Done | ✅ PASS w/ notes | ✅ | ✅ 8a7bed4 |
-| Phase 2: Split sections | frontend | ✅ Done | ⬜ pending | ✅ node --check | ✅ 92030b4 |
-| Phase 3: Dashboard | fullstack | ⬜ Not Started | ⬜ | ⬜ | ⬜ |
+| Phase 2: Split sections | frontend | ✅ Done | ✅ PASS (1 blocker fixed: fa67ad1) | ✅ node --check | ✅ 92030b4 + fa67ad1 |
+| Phase 3: Dashboard | fullstack | ✅ Done | ⬜ pending | ✅ node --check + queries verified | ⬜ |
 | Phase 4: Palette | fullstack | ⬜ Not Started | ⬜ | ⬜ | ⬜ |
 | Phase 5: Quick actions | frontend | ⬜ Not Started | ⬜ | ⬜ | ⬜ |
 | Phase 6: Deep pages | frontend | ⬜ Not Started | ⬜ | ⬜ | ⬜ |