fix(theme): apply per-user theme_pref on login, persist toggle to server

alpine-init.js resolved the theme purely from localStorage, so it never
reflected the logged-in user's theme_pref: switching user kept the previous
user's theme because localStorage persists per-browser.

- base.html injects window.__SERVER_THEME from the session theme (empty when
  anonymous, preserving OS-preference behaviour on the login page).
- alpine-init.js now treats the server value as authoritative for logged-in
  users (server > localStorage > OS > light) and syncs localStorage to it.
- The navbar toggle now persists the choice via POST /auth/set-theme, which
  updates the session and the user's theme_pref, so it survives navigation
  and matches on the next login.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/frontend/flask_app/templates/base.html

@@ -24,6 +24,9 @@
   <!-- Theme CSS Variables -->
   <link rel="stylesheet" href="{{ url_for('static', filename='css/themes.css') }}">
 
+  <!-- Authoritative per-user theme from the server session (empty when anonymous) -->
+  <script>window.__SERVER_THEME = "{{ current_theme if current_user else '' }}";</script>
+
   <!-- Alpine.js Theme Init (before Alpine loads) -->
   <script src="{{ url_for('static', filename='js/alpine-init.js') }}"></script>