feat(admin): phase 1 — hash-router

AdminRouter wraps existing switchTab for deep-linking.

- frontend/js/admin/router.js (new, 102L): parse/navigate/current/on/off, recursion guard via _navigating flag

- admin.html: +1 <script> before admin.js

- admin.js: switchTab(btn, opts) + initAdminRouter IIFE for hashchange dispatch

Backward compat: all 21 onclick=switchTab(this) callsites continue working.

F5 / back / forward / deep-link verified.

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

frontend/admin.html

@@ -1981,6 +1981,7 @@
 
   <script src="/js/api.js"></script>
   <script src="/js/sidebar.js"></script>
+  <script src="/js/admin/router.js"></script>
   <script src="/js/admin/admin.js"></script>
   </div>
 </div>