feat(ui): implement 'Twilight Velvet' dark theme and fix visibility issues

- Add 'Twilight Velvet' color palette to globals.css with OKLCH values
- Update SettingsPage headers, cards, and dialogs to use semantic theme variables
- Update HistoryCard, HistoryFeed, and DraftContent to support dark mode
- Update ProviderSelector and ProviderList to use custom card background (#2A2A3D)
- Add ThemeToggle component with improved visibility
- Ensure consistent use of 'bg-card', 'text-foreground', and 'text-muted-foreground'

package.json

@@ -15,6 +15,7 @@
     "@ai-sdk/openai-compatible": "^2.0.18",
     "@radix-ui/react-alert-dialog": "^1.1.15",
     "@radix-ui/react-dialog": "^1.1.15",
+    "@radix-ui/react-dropdown-menu": "^2.1.16",
     "@radix-ui/react-label": "^2.1.8",
     "@radix-ui/react-slot": "^1.2.4",
     "@testing-library/user-event": "^14.6.1",
@@ -27,6 +28,7 @@
     "lucide-react": "^0.562.0",
     "next": "16.1.4",
     "next-pwa": "^5.6.0",
+    "next-themes": "^0.4.6",
     "react": "19.2.3",
     "react-dom": "19.2.3",
     "react-markdown": "^10.1.0",