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/static/js/alpine-init.js

@@ -16,9 +16,21 @@
 
   /**
    * Resolve the initial dark-mode state.
-   * Priority: localStorage > system preference > light (default).
+   * Priority: server (per-user theme_pref) > localStorage > system preference > light.
+   *
+   * The server value (window.__SERVER_THEME) is authoritative for logged-in
+   * users: it reflects the theme_pref of whoever is currently authenticated,
+   * so switching user actually switches theme instead of inheriting the
+   * previous user's localStorage value. It is empty for anonymous pages.
    */
   function getInitialDark() {
+    var server = (typeof window !== 'undefined' && window.__SERVER_THEME) || '';
+    if (server === 'dark' || server === 'light') {
+      // Keep localStorage in sync so the rest of the app stays consistent.
+      try { localStorage.setItem(STORAGE_KEY, server); } catch (_) {}
+      return server === 'dark';
+    }
+
     var stored = null;
     try {
       stored = localStorage.getItem(STORAGE_KEY);
@@ -70,6 +82,7 @@
       toggle: function () {
         this.dark = !this.dark;
         this._apply();
+        this._persist();
       },
 
       set: function (dark) {
@@ -77,6 +90,24 @@
         this._apply();
       },
 
+      // Persist an explicit user choice to the server (session + theme_pref),
+      // so it survives navigation and matches on the next login. Best-effort.
+      _persist: function () {
+        try {
+          var csrf = document.querySelector('meta[name=csrf-token]');
+          fetch('/auth/set-theme', {
+            method: 'POST',
+            headers: {
+              'Content-Type': 'application/json',
+              'X-CSRFToken': (csrf && csrf.content) || ''
+            },
+            body: JSON.stringify({ theme: this.dark ? 'dark' : 'light' })
+          }).catch(function () {});
+        } catch (_) {
+          // Non-fatal: localStorage already holds the choice for this browser.
+        }
+      },
+
       _apply: function () {
         if (this.dark) {
           document.documentElement.classList.add('dark');