feat: add JSON with Images export/import format

[JustCreature]

JSON with Images Export/Import (F-2)

Single-file export format that embeds images as base64. Simplifies sharing via messaging apps.

Changes

• New export method: "JSON with Images" creates single JSON file
• Import from main screen: Import button in film rolls toolbar
• Auto-navigation: Import redirects to gallery, no reload needed
• Size warnings: Alert if export >10MB
• [IMPORTED] prefix: Distinguishes imported film rolls

Files Changed

• src/types.ts - New export format types
• src/utils/exportImport.ts - Export/import methods (~140 lines)
• src/components/FilmRollListScreen.tsx - Import UI (~120 lines)
• src/components/GalleryScreen.tsx - Export UI (~50 lines)
• src/App.tsx, src/components/MainScreen.tsx - State wiring

Testing

• 8 E2E tests in e2e/import-export.spec.ts
• Tests import flow, navigation, metadata preservation
• Cross-browser (5 browsers)

User Benefits

✅ Single file to share (Telegram, WhatsApp, email)
✅ No file loss (images embedded)
✅ Instant feedback (auto-navigation)
✅ Backward compatible (existing exports work)