feat: add update instructions dialog, email notifications, and capture date sorting

- Add Update Instructions Dialog with environment-specific commands (Docker/Git/Standalone)
- Add email notification settings for new version alerts
- Add "Sort by Capture Date" option using EXIF metadata extraction
- Fix E2E tests by loading environment variables via dotenv
- Add test-images/ and backend/*.db to .gitignore

Closes #181

frontend/src/types/index.ts

@@ -81,6 +81,7 @@ export interface Photo {
   category_slug?: string;
   size: number;
   uploaded_at: string;
+  captured_at?: string; // EXIF capture date (if available)
   // Media type fields
   media_type?: 'photo' | 'video' | 'image';
   mime_type?: string;