feat(frontend): implement file gallery page with infinite scroll

Adds InfiniteScroll component (IntersectionObserver, 300px margin,
CSS spinner). Adds FileCard component (fetch thumbnail with JWT auth
header, blob URL, shimmer placeholder). Adds files/+page.svelte with
160×160 flex-wrap grid and cursor pagination. Updates mock plugin with
75 sample files, cursor pagination, and colored SVG thumbnail handler.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/package.json

@@ -18,6 +18,7 @@
 		"@sveltejs/kit": "^2.50.2",
 		"@sveltejs/vite-plugin-svelte": "^6.2.4",
 		"@tailwindcss/vite": "^4.2.2",
+		"@types/node": "^25.5.2",
 		"openapi-typescript": "^7.13.0",
 		"svelte": "^5.54.0",
 		"svelte-check": "^4.4.2",