Fix language setting not being saved to database on admin settings page

- Added default_language field to general settings state in SettingsPage
- Replaced LanguageSelector component with simple select dropdown on settings page
- Fixed public settings endpoint to read general_default_language from database
- Language setting now properly saved when clicking Save Settings button
- Setting is correctly used by gallery login page and legal pages

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/config/api.ts

@@ -26,6 +26,11 @@ api.interceptors.request.use(
       config.headers.Authorization = `Bearer ${token}`;
     }
 
+    // Don't set Content-Type for FormData - let browser set it with boundary
+    if (config.data instanceof FormData) {
+      delete config.headers['Content-Type'];
+    }
+
     return config;
   },
   (error) => {