feat: move theme toggle to settings dialog

Move the theme toggle from the main header to the settings dialog for a
cleaner UI and better organization. The new theme selector uses a
button group design consistent with the language settings.

Changes:
- Created ThemeSettings component with three options: Light, Dark, System
- Added ThemeSettings to the General tab in settings dialog
- Removed ModeToggle component from main header
- Added theme-related i18n keys for all options
- Theme selection takes effect immediately without requiring save

Design:
- Uses button group style matching LanguageSettings
- Icons for each theme option (Sun, Moon, Monitor)
- Consistent with app's blue theme for active state
- Smooth transitions and hover effects

This change simplifies the main header and consolidates all appearance
settings in one place, improving the overall user experience.

src/App.tsx

@@ -8,7 +8,6 @@ import { providersApi, settingsApi, type AppType, type ProviderSwitchEvent } fro
 import { useProviderActions } from "@/hooks/useProviderActions";
 import { extractErrorMessage } from "@/utils/errorUtils";
 import { AppSwitcher } from "@/components/AppSwitcher";
-import { ModeToggle } from "@/components/mode-toggle";
 import { ProviderList } from "@/components/providers/ProviderList";
 import { AddProviderDialog } from "@/components/providers/AddProviderDialog";
 import { EditProviderDialog } from "@/components/providers/EditProviderDialog";
@@ -117,7 +116,6 @@ function App() {
             >
               CC Switch
             </a>
-            <ModeToggle />
             <Button
               variant="ghost"
               size="icon"