feat(admin): Phase 6 sub-commit 1 — add deep-page sections (overlay still works)

Add user-detail.js (~370L) and session-detail.js (~180L) section
modules that render full pages for #users/:id and #sessions/:id, plus
admin.js dispatch and HTML tab-panes. The legacy .user-panel overlay
is intentionally still in place — sub-commit 2 will remove it once the
deep pages are verified.

* admin.js: DEEP_ROUTES map + activateDeepPane(); activate(route, params)
  signature; initial dispatch respects hash params (so F5 on #users/123
  goes straight to the deep page).
* admin.html: new tab-panes #tab-user-detail / #tab-session-detail and
  two script tags. Old #user-panel overlay untouched.
* user-detail.js: header (avatar/role/email/meta) + sub-tabs
  (Обзор/Сессии/Классы/Audit) with URL-synced sub-tab routing
  (#users/N/sessions etc). Overview: 4 stat cards + per-subject SVG
  bar chart. Sessions: clickable rows that navigate to #sessions/N.
  Classes: placeholder empty-state (no per-user classes endpoint).
  Audit: client-side filter of /admin/audit-log by uid match. Header
  action buttons (Изменить/Права/История/Бан/Удалить) call existing
  overlay handlers; window.activeUid is set before opening any modal.
* session-detail.js: full header (user/subject/score/stats) + per-
  question correctness layout reusing the drawer renderer. Delete
  button uses LS.adminDeleteSession then navigates to #sessions.
  Clicking the user name opens the user deep page.
* users.js: quickOpenUserSessions now navigates to
  #users/<uid>/sessions instead of the bare #sessions list.

Verified node --check on all new/modified JS. baseline npm test still
shows pre-existing 3 auth failures unrelated to this change.

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

plans/admin-redesign/phase-6-deep-pages.md

@@ -1,6 +1,6 @@
 # Phase 6: Deep entity pages
 
-**Status:** ⬜ Not Started
+**Status:** 🟡 In Progress (sub-commit 1 of 2 done)
 **Parent plan:** [PLAN.md](./PLAN.md)
 **Domain:** frontend