feat(ui): internationalize profile, subscription, integrations templates and user menu JS

- Add 155 new translation keys to en.json (common, nav, integrations, profile, subscription sections)
- Add window.__i18n script block to base.html for user menu strings
- Replace all hardcoded English strings in common.js with window.__i18n lookups using safe fallback pattern
- Internationalize profile.html (page title, headings, labels, hints, placeholders, theme options)
- Internationalize subscription.html (page title, plan cards, usage stats, plan actions, badges)
- Internationalize integrations_dashboard.html (header, quota bars, modal, all form fields, action buttons, webhook section, delete modal)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

frontend/templates/base.html

@@ -548,6 +548,23 @@
       })();
     </script>
 
+    <!-- i18n strings for common.js (user menu) -->
+    <script>
+      window.__i18n = {
+        accountMenu: {{ _("nav.account_menu") | tojson }},
+        accountMenuFor: {{ _("nav.account_menu_for") | tojson }},
+        avatar: {{ _("common.avatar") | tojson }},
+        profileSettings: {{ _("nav.profile_settings") | tojson }},
+        mySubscription: {{ _("nav.my_subscription") | tojson }},
+        apiTokens: {{ _("nav.api_tokens") | tojson }},
+        sharedLinks: {{ _("nav.shared_links") | tojson }},
+        signOut: {{ _("nav.sign_out") | tojson }},
+        logIn: {{ _("nav.login") | tojson }},
+        signUp: {{ _("nav.signup") | tojson }},
+        getStarted: {{ _("nav.get_started") | tojson }},
+      };
+    </script>
+
     <!-- Common JS (shared) -->
     <script src="/static/js/common.js"></script>