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'

playwright.config.ts

@@ -2,7 +2,7 @@ import { defineConfig, devices } from '@playwright/test';
 
 export default defineConfig({
     testDir: './tests',
-    testIgnore: '**/component/**',
+    testIgnore: ['**/component/**', '**/unit/**'],
     fullyParallel: true,
     forbidOnly: !!process.env.CI,
     retries: process.env.CI ? 2 : 0,