Add bookmark feature to pin webpages to desktop

## 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