build(frontend): type-check the production build, pin knip

`npm run build` now runs `typecheck` first, so the Docker image is built through the
same gate as local dev — until now the image was a bare `vite build`, and esbuild
strips types without checking them.

typecheck covers three projects: src, the Node-side build configs (new
tsconfig.node.json — vite.config.ts/vitest.config.ts were type-checked by nothing),
and e2e. knip moves from `npx knip@latest` to a pinned devDependency so the gate
stops depending on the network and on whichever version shipped that morning.

frontend/package.json

@@ -4,10 +4,13 @@
   "type": "module",
   "scripts": {
     "dev": "vite",
-    "build": "vite build",
+    "typecheck": "tsc --noEmit -p tsconfig.json && tsc --noEmit -p tsconfig.node.json && tsc --noEmit -p e2e/tsconfig.json",
+    "build": "npm run typecheck && vite build",
     "preview": "vite preview",
+    "knip": "knip",
     "test": "vitest run",
-    "test:watch": "vitest"
+    "test:watch": "vitest",
+    "e2e": "playwright test"
   },
   "dependencies": {
     "@dnd-kit/core": "^6.3.1",
@@ -31,6 +34,7 @@
     "@types/react-dom": "^18.3.0",
     "@vitejs/plugin-react": "^4.3.1",
     "autoprefixer": "^10.4.19",
+    "knip": "6.27.0",
     "postcss": "^8.4.39",
     "tailwindcss": "^3.4.6",
     "typescript": "^5.5.3",