Add bookmark feature to pin webpages to desktop

[danperks]

Overview

This PR adds a bookmark feature that allows users to pin external webpages to their umbrelOS desktop.

Features

• Add bookmarks via desktop context menu
• Auto-fetch metadata: Automatically fetches page title and favicon from URLs
• Custom icons: Upload custom icons or use auto-fetched favicons
• Smart backgrounds: Detects icon edge colors and fills transparent backgrounds
• Edit/Delete: Right-click bookmarks to edit or delete
• External indicator: Visual indicator to distinguish bookmarks from apps
• New tab support: Optional checkbox to open links in new tab
• i18n: Full translations for all 12 supported locales

Technical Details

• Backend: Added TRPC routes for bookmark CRUD operations
• Storage: Bookmarks stored in user preferences
• CORS handling: Favicon/title requests proxied through backend
• CSP: Added data: URLs to img-src for base64 icons
• ID generation: Compact base36 timestamp IDs