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'

.env.example

@@ -14,3 +14,5 @@ FEATURE_FLAG_NEW_UI=true
 OPENAI_API_KEY=your_openai_api_key_here
 LLM_MODEL=gpt-4o-mini
 LLM_TEMPERATURE=0.7
+# Security
+APP_PASSWORD=password