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>

frontend/admin.html

@@ -1143,6 +1143,16 @@
       </div>
     </div>
 
+    <!-- ── Deep page: user detail (#users/:id) — populated by user-detail.js ── -->
+    <div class="tab-pane" id="tab-user-detail">
+      <div id="user-detail-content"><div class="spinner"></div></div>
+    </div>
+
+    <!-- ── Deep page: session detail (#sessions/:id) — populated by session-detail.js ── -->
+    <div class="tab-pane" id="tab-session-detail">
+      <div id="session-detail-content"><div class="spinner"></div></div>
+    </div>
+
     <!-- ── Тесты (сессии) ── -->
     <div class="tab-pane" id="tab-sessions">
       <div class="t-toolbar">
@@ -2006,6 +2016,8 @@
   <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/sections/user-detail.js"></script>
+  <script src="/js/admin/sections/session-detail.js"></script>
   <script src="/js/admin/palette.js"></script>
   <script src="/js/admin/admin.js"></script>
   </div>