feat: add working theme selector and improve settings UI

Added fully functional theme selector and improved the settings UI with
better components and cleaner copy.

Changes:
- Created Switch component (shadcn/radix-ui) for boolean settings
- Added working theme selector that integrates with existing theme system
- Uses useTheme hook to display available themes (Dark, Light, Plan9)
- Reordered sections: Appearance first, Post second
- Reordered settings: Theme first in Appearance section
- Replaced Checkbox with Switch for better UX on boolean toggles
- Simplified copy: "Add Grimoire tag to published events" instead of listing kinds
- Simplified copy: "Display client identifiers in events" instead of "via Grimoire" mention
- Better layout: Label/description on left, Switch on right

Settings now use proper form components:
- Switch for boolean toggles (include client tag, show client tags)
- Button group for theme selection
- Clean justify-between layout for settings rows

The theme selector works immediately - clicking Dark/Light/Plan9 applies
the theme instantly via the existing ThemeProvider context.

package.json

@@ -30,6 +30,7 @@
     "@radix-ui/react-scroll-area": "^1.2.10",
     "@radix-ui/react-slider": "^1.3.6",
     "@radix-ui/react-slot": "^1.2.4",
+    "@radix-ui/react-switch": "^1.2.6",
     "@radix-ui/react-tabs": "^1.1.13",
     "@radix-ui/react-tooltip": "^1.2.8",
     "@tiptap/core": "^3.15.3",