feat: add React frontend and serve it from Express

Also fixes a Windows-specific bug in worker.js and cleanup.js: the
"run only if executed directly" guard compared import.meta.url against
`file://${process.argv[1]}`, which never matches on Windows (backslash
path separators, missing extra slash before the drive letter). main()
silently never ran, so the worker process started and exited
immediately without ever polling. Caught during this task's manual
verification of the full upload-convert-download flow. Fixed with
node:url's pathToFileURL, which builds a correct file:// URL cross-platform.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

package.json

@@ -7,7 +7,8 @@
     "start": "node src/server.js",
     "worker": "node src/worker.js",
     "cleanup": "node src/cleanup.js",
-    "test": "vitest run"
+    "test": "vitest run",
+    "build": "npm install --prefix frontend && npm run build --prefix frontend"
   },
   "dependencies": {
     "docx": "^9.7.1",