feat(03-04): ImageGallery and ImageLightbox components

- ImageGallery renders horizontal scrolling thumbnails
- Edit mode shows delete buttons on images
- ImageLightbox provides fullscreen image viewer
- Keyboard navigation (Escape, arrows) in lightbox
- Click outside image or X button closes lightbox
- Image counter shown when multiple images

package.json

@@ -37,6 +37,7 @@
 		"nanoid": "^5.1.6",
 		"sharp": "^0.34.5",
 		"svelte-gestures": "^5.2.2",
+		"svelte-lightbox": "^1.1.7",
 		"svelte-persisted-store": "^0.12.0",
 		"tailwindcss": "^4.1.18",
 		"zod": "^4.3.6"