feat(profile): expose default_document_language in profile API and UI

- Add default_document_language to ProfileResponse and ProfileUpdateRequest
- Handle validation in PATCH /api/profile endpoint
- Add dropdown in profile.html template with Alpine.js binding
- Add translation keys for profile UI labels
- Add comprehensive tests for profile default language feature

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

frontend/templates/profile.html

@@ -231,6 +231,28 @@
           {{ _("profile.theme_hint") }}
         </p>
       </fieldset>
+
+      <!-- Default Document Language -->
+      <div>
+        <label for="doc-lang-select" class="block text-sm font-medium text-gray-700 dark:text-gray-300 mb-1">
+          <i class="fas fa-language text-gray-400 mr-1" aria-hidden="true"></i>{{ _("profile.default_document_language_label") }}
+        </label>
+        <select
+          id="doc-lang-select"
+          x-model="form.default_document_language"
+          class="w-full px-3 py-2 border border-gray-300 dark:border-gray-600 rounded-md shadow-sm text-sm
+                 focus:outline-none focus:ring-2 focus:ring-blue-500 dark:bg-gray-700 dark:text-white"
+          style="min-height:44px;"
+        >
+          <option value="">{{ _("profile.default_document_language_auto") }}</option>
+          {% for lang in supported_languages %}
+          <option value="{{ lang.code }}">{{ lang.flag }} {{ lang.native }} ({{ lang.name }})</option>
+          {% endfor %}
+        </select>
+        <p class="mt-1 text-xs text-gray-500 dark:text-gray-400">
+          {{ _("profile.default_document_language_hint") }}
+        </p>
+      </div>
     </div>
   </section>
 
@@ -340,6 +362,7 @@ function profileSettings() {
       contact_email: '',
       preferred_language: '',
       preferred_theme: 'system',
+      default_document_language: '',
     },
 
     pwForm: {
@@ -363,6 +386,7 @@ 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.form.default_document_language = data.default_document_language || '';
         this._initialLanguage = this.form.preferred_language;
       } catch (_e) {
         // Silently ignore — user might not be logged in (rare for this page)

frontend/translations/en.json

@@ -1377,6 +1377,9 @@
   "profile.contact_email_label": "Contact / Notification E-mail",
   "profile.contact_email_placeholder": "you@example.com",
   "profile.current_password": "Current Password",
+  "profile.default_document_language_auto": "Use system default",
+  "profile.default_document_language_hint": "Documents in other languages are automatically translated into this language. Leave blank to use the system default (English).",
+  "profile.default_document_language_label": "Default Document Language",
   "profile.dismiss": "Dismiss",
   "profile.display_name_hint": "Leave blank to use your account username or email.",
   "profile.display_name_label": "Display Name",