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-lock.json

@@ -13,6 +13,7 @@
 				"@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",
@@ -1345,6 +1346,16 @@
 			"dev": true,
 			"license": "MIT"
 		},
+		"node_modules/@types/node": {
+			"version": "25.5.2",
+			"resolved": "https://registry.npmjs.org/@types/node/-/node-25.5.2.tgz",
+			"integrity": "sha512-tO4ZIRKNC+MDWV4qKVZe3Ql/woTnmHDr5JD8UI5hn2pwBrHEwOEMZK7WlNb5RKB6EoJ02gwmQS9OrjuFnZYdpg==",
+			"dev": true,
+			"license": "MIT",
+			"dependencies": {
+				"undici-types": "~7.18.0"
+			}
+		},
 		"node_modules/@types/trusted-types": {
 			"version": "2.0.7",
 			"resolved": "https://registry.npmjs.org/@types/trusted-types/-/trusted-types-2.0.7.tgz",
@@ -2453,6 +2464,13 @@
 				"node": ">=14.17"
 			}
 		},
+		"node_modules/undici-types": {
+			"version": "7.18.2",
+			"resolved": "https://registry.npmjs.org/undici-types/-/undici-types-7.18.2.tgz",
+			"integrity": "sha512-AsuCzffGHJybSaRrmr5eHr81mwJU3kjw6M+uprWvCXiNeN9SOGwQ3Jn8jb8m3Z6izVgknn1R0FTCEAP2QrLY/w==",
+			"dev": true,
+			"license": "MIT"
+		},
 		"node_modules/uri-js-replace": {
 			"version": "1.0.1",
 			"resolved": "https://registry.npmjs.org/uri-js-replace/-/uri-js-replace-1.0.1.tgz",