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/components/settings/SettingsDialog.tsx

@@ -12,6 +12,7 @@ import { Tabs, TabsContent, TabsList, TabsTrigger } from "@/components/ui/tabs";
 import { Button } from "@/components/ui/button";
 import { settingsApi } from "@/lib/api";
 import { LanguageSettings } from "@/components/settings/LanguageSettings";
+import { ThemeSettings } from "@/components/settings/ThemeSettings";
 import { WindowSettings } from "@/components/settings/WindowSettings";
 import { DirectorySettings } from "@/components/settings/DirectorySettings";
 import { ImportExportSection } from "@/components/settings/ImportExportSection";
@@ -201,6 +202,7 @@ export function SettingsDialog({
                       value={settings.language}
                       onChange={(lang) => updateSettings({ language: lang })}
                     />
+                    <ThemeSettings />
                     <WindowSettings
                       settings={settings}
                       onChange={updateSettings}