fix(i18n): persist language preference server-side, fix dropdown rendering and avatar 404

- Fix language dropdown in base.html by moving tojson data out of HTML
  attribute into a script tag (prevents quote conflicts breaking Alpine.js)
- Fix avatar placeholder 404 by correcting filename reference from
  avatar-placeholder.svg to default-avatar.svg
- Add session hydration from DB in _inject_global_context() so
  detect_language() uses the stored preference on every request
- Sync session and cookie in PATCH /api/profile when language changes
- Reload page after language change in profile to reflect new locale
- Add tests for session/cookie sync and DB hydration

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

frontend/templates/base.html

@@ -216,12 +216,18 @@
                     aria-live="polite"></span>
             </a>
 
+            <!-- Language selector data (kept outside the HTML attribute to avoid quote conflicts with tojson) -->
+            <script>
+              window.__langSuggested = {{ suggested_languages | tojson }};
+              window.__langAll = {{ supported_languages | tojson }};
+            </script>
+
             <!-- Language selector dropdown -->
             <div x-data="{
               langOpen: false,
               search: '',
-              suggested: {{ suggested_languages | tojson }},
-              all: {{ supported_languages | tojson }},
+              suggested: window.__langSuggested,
+              all: window.__langAll,
               get filtered() {
                 if (!this.search.trim()) return this.suggested;
                 const q = this.search.trim().toLowerCase();

frontend/templates/profile.html

@@ -327,7 +327,7 @@
 function profileSettings() {
   return {
     // ── State ──────────────────────────────────────────────────────────────
-    avatarUrl: '/static/images/avatar-placeholder.svg',
+    avatarUrl: '/static/images/default-avatar.svg',
     hasCustomAvatar: false,
     isLocalUser: false,
     saving: false,
@@ -349,6 +349,8 @@ function profileSettings() {
     },
 
     // ── Init ───────────────────────────────────────────────────────────────
+    _initialLanguage: '',
+
     async init() {
       try {
         const res = await fetch('/api/profile');
@@ -361,6 +363,7 @@ function profileSettings() {
         this.form.contact_email    = data.contact_email    || '';
         this.form.preferred_language = data.preferred_language || '';
         this.form.preferred_theme    = data.preferred_theme  || 'system';
+        this._initialLanguage = this.form.preferred_language;
       } catch (_e) {
         // Silently ignore — user might not be logged in (rare for this page)
       }
@@ -378,6 +381,7 @@ function profileSettings() {
     async saveProfile() {
       this.saving = true;
       this._hideBanner();
+      const previousLang = this._initialLanguage || '';
       try {
         const csrfToken = this._getCSRFToken();
 
@@ -394,6 +398,11 @@ function profileSettings() {
           this._showBanner(data.detail || 'Failed to save settings.', true);
           return;
         }
+        // If the UI language changed, reload so the page renders in the new locale
+        if (this.form.preferred_language !== previousLang) {
+          window.location.reload();
+          return;
+        }
         this._showBanner('Profile settings saved successfully.', false);
         // Update display name across the nav (refresh the avatar/name)
         if (data.avatar_url) {