feat(mobile): Custom Photo Picker with Multi-Select

Problem

The native iOS PHPicker (launched via expo-image-picker) is slow to display photos on the occasion details screen.

Solution

Build a custom in-app photo picker using expo-media-library with FlashList for optimal performance.

Requirements

• No cropping - Direct selection without edit screen
• Multi-select - Allow selecting multiple photos at once
• Progressive thumbnail loading (fast initial display)
• Album selection (All Photos, Camera Roll, Screenshots, etc.)
• Camera option still uses fast expo-image-picker

Files to Create

• mobile/src/components/photo-picker/PhotoPickerSheet.tsx - Main container
• mobile/src/components/photo-picker/PhotoGrid.tsx - FlashList grid
• mobile/src/components/photo-picker/PhotoGridItem.tsx - Thumbnails with selection
• mobile/src/components/photo-picker/AlbumSelector.tsx - Album dropdown
• mobile/src/components/photo-picker/index.ts - Public exports
• mobile/src/hooks/useMediaLibrary.ts - Media library hook

Files to Modify

• mobile/src/components/celebration/MemoryUploadButton.tsx - Use new picker + batch upload
• mobile/src/components/PhotoUploadButton.tsx - Use new picker
• mobile/app.config.js - Add expo-media-library plugin
• mobile/src/locales/*/photoPicker.json - i18n translations (7 languages)

Technical Details

• Grid: 3 columns, ~110-120px thumbnails
• Pagination: 50 photos per page with cursor-based pagination
• Selection: Numbered badges showing selection order
• Permission flow: Handle granted/limited/denied states

Acceptance Criteria

• [ ] Custom picker loads faster than native PHPicker
• [ ] Multi-select works with numbered selection badges
• [ ] Album switching works
• [ ] Camera button launches expo-image-picker camera
• [ ] Works on both iOS and Android
• [ ] i18n translations for all 7 languages
• [ ] Unit tests for components