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/tsconfig.json

@@ -1,5 +1,6 @@
 {
 	"extends": "./.svelte-kit/tsconfig.json",
+	"exclude": ["vite-mock-plugin.ts"],
 	"compilerOptions": {
 		"rewriteRelativeImportExtensions": true,
 		"allowJs": true,