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'

_bmad-output/project-context.md

@@ -1,5 +1,5 @@
 ---
-project_name: 'Brachnha Insights'
+project_name: 'Brachnha'
 user_name: 'Max'
 date: '2026-01-21'
 sections_completed: ['technology_stack', 'implementation_rules', 'naming_conventions', 'project_structure']