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/index.css

@@ -160,4 +160,39 @@
   .smooth-scroll {
     scroll-behavior: smooth;
   }
+
+  /* Custom range slider styles */
+  .slider {
+    -webkit-appearance: none;
+    appearance: none;
+    background: transparent;
+    cursor: pointer;
+  }
+
+  .slider::-webkit-slider-track {
+    @apply bg-neutral-200 h-2 rounded-lg;
+  }
+
+  .slider::-moz-range-track {
+    @apply bg-neutral-200 h-2 rounded-lg;
+  }
+
+  .slider::-webkit-slider-thumb {
+    -webkit-appearance: none;
+    appearance: none;
+    @apply bg-primary-600 h-5 w-5 rounded-full cursor-pointer transition-all;
+    margin-top: -6px;
+  }
+
+  .slider::-moz-range-thumb {
+    @apply bg-primary-600 h-5 w-5 rounded-full cursor-pointer transition-all border-0;
+  }
+
+  .slider:hover::-webkit-slider-thumb {
+    @apply bg-primary-700 scale-110;
+  }
+
+  .slider:hover::-moz-range-thumb {
+    @apply bg-primary-700 scale-110;
+  }
 }