refactor: improve settings UI with Select, icons, and better spacing

Enhanced the settings UI with shadcn Select component, tab icons, and
improved typography and spacing.

Changes:
- Created Select component (shadcn/radix-ui) for dropdowns
- Added icons to tabs: Palette for Appearance, FileEdit for Post
- Replaced button group theme selector with Select dropdown (w-48)
- Added gap-4 between labels and switches for better spacing
- Increased setting names from text-sm to text-base (bigger)
- Reduced help text from text-sm to text-xs (smaller)
- Added gap-3 between "Theme" label and Select
- Added font-medium to "Theme" label for consistency

Layout improvements:
- Theme selector now uses Select with 192px width
- All setting rows have gap-4 between content and controls
- Consistent text hierarchy: font-medium for labels, text-xs for descriptions
- Tab triggers have gap-2 between icon and text

The Select component is properly integrated with the theme system and
displays Dark, Light, and Plan9 options with checkmark indicators.

package.json

@@ -28,6 +28,7 @@
     "@radix-ui/react-popover": "^1.1.15",
     "@radix-ui/react-progress": "^1.1.8",
     "@radix-ui/react-scroll-area": "^1.2.10",
+    "@radix-ui/react-select": "^2.2.6",
     "@radix-ui/react-slider": "^1.3.6",
     "@radix-ui/react-slot": "^1.2.4",
     "@radix-ui/react-switch": "^1.2.6",