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'

tests/support/factories/settings.factory.ts

@@ -0,0 +1,20 @@
+import { faker } from '@faker-js/faker';
+
+export interface UserSettings {
+    theme: 'light' | 'dark' | 'system';
+    fontSize: number;
+    reduceMotion: boolean;
+    language: string;
+    notificationsEnabled: boolean;
+}
+
+export const createSettings = (overrides: Partial<UserSettings> = {}): UserSettings => {
+    return {
+        theme: faker.helpers.arrayElement(['light', 'dark', 'system']),
+        fontSize: faker.number.int({ min: 12, max: 24 }),
+        reduceMotion: faker.datatype.boolean(),
+        language: 'en-US',
+        notificationsEnabled: faker.datatype.boolean(),
+        ...overrides,
+    };
+};