feat: add pack cover image preview and upload in Pack Editor Drawer

Commit notes

- Add uploadPackCover API function to frontend/src/lib/api/packs.ts - Add createUploadPackCoverMutation hook to frontend/src/lib/query/hooks/packs.ts - Export the new mutation from frontend/src/lib/query/index.ts - Update PackEditDrawer.svelte with: - Cover image preview display (shows current cover or fallback gradient) - Click-to-upload functionality with camera icon overlay on hover - File type validation (JPEG, PNG, WebP) - File size validation (max 10MB) - Upload progress spinner indicator - Error handling and display - Reset state when pack changes

Co-authored-by: armin.naimi <[email redacted]>