fix(ui): drive Vercel dashboard-API proxy through Nitro routeRules (#76)

The Vercel deploy 404'd at `/` because PR #75's `vercel-build` script
(`scripts/build-vercel-output.mjs`) ran `rm -rf .vercel/output` and rebuilt
it from `.output/public`. On Vercel CI, Nitro's Vercel preset auto-activates
(from the VERCEL env var) and emits the Build Output API layout to
`.vercel/output` itself during `vite build`; the script then clobbered that
correct output with a static-only config that could not resolve the SPA
`_shell.html` fallback or the server functions, so production returned
`404: NOT_FOUND` even though the build was READY.

Stop fighting Nitro and drive the proxy through it instead:

- Remove `scripts/build-vercel-output.mjs` and the `vercel-build` script;
  restore the plain `vite build` for both local and Vercel builds.
- Add an env-driven Nitro `routeRules` proxy in `vite.config.ts`. Nitro's
  Vercel preset compiles a plain external-URL `proxy` rule into a CDN-level
  rewrite in `.vercel/output/config.json` at build time, reading
  LANGGRAPH_BACKEND_URL (the per-project Vercel env var). Proxy, not redirect,
  so the osw_session cookie stays first-party (same-origin). The build fails
  loudly if LANGGRAPH_BACKEND_URL is missing on Vercel; the rule is omitted
  for plain local/off-Vercel builds (which use the E2E_HARNESS mock proxy).
- `vercel.json`: `buildCommand` back to `bun run build`; drop the
  `.output/public` outputDirectory so Vercel serves Nitro's `.vercel/output`.

Validated with `LANGGRAPH_BACKEND_URL=... NITRO_PRESET=vercel bun run build`:
the generated `.vercel/output/config.json` contains the
`/dashboard/api/(.*)` -> backend rewrite ahead of `handle: filesystem` and
the SPA catch-all, `_shell.html` and the 699 hashed assets are emitted, and
typecheck passes. Supersedes the broken approach in #75.

ui/vercel.json

@@ -1,15 +1,5 @@
 {
+  "$schema": "https://openapi.vercel.sh/vercel.json",
   "framework": null,
-  "buildCommand": "bun run build",
-  "outputDirectory": ".output/public",
-  "rewrites": [
-    {
-      "source": "/dashboard/api/:path*",
-      "destination": "https://open-swe-v3-f2834ffc0df05a46a10262a9690e8490.us.langgraph.app/dashboard/api/:path*"
-    },
-    {
-      "source": "/(.*)",
-      "destination": "/_shell.html"
-    }
-  ]
+  "buildCommand": "bun run build"
 }

ui/vite.config.ts

@@ -74,13 +74,44 @@ function mockHarnessProxy(): Plugin | null {
   }
 }
 
+// Same-origin proxy for the dashboard API. Vercel reads vercel.json BEFORE the build
+// and never interpolates env into it, and prod promotes by fast-forward (so `dev` and
+// `main` ship a byte-identical vercel.json that cannot encode a per-project backend).
+// So the per-project backend is driven through Nitro instead: this `routeRules` proxy
+// is compiled by Nitro's Vercel preset into a CDN-level rewrite in
+// `.vercel/output/config.json` at build time, reading LANGGRAPH_BACKEND_URL from the
+// per-project Vercel env var. Proxy (not redirect) keeps the osw_session cookie
+// first-party on the Vercel origin. The `**` wildcard maps 1:1 to the destination.
+function backendProxyRouteRules():
+  | Record<string, { proxy: string }>
+  | undefined {
+  const backend = process.env.LANGGRAPH_BACKEND_URL?.replace(/\/$/, "")
+  // Nitro's Vercel preset auto-activates from the VERCEL env var; NITRO_PRESET=vercel
+  // forces it locally. On Vercel the backend is mandatory — fail loudly rather than
+  // ship a build whose API calls 404, which is exactly how the previous approach broke.
+  const onVercel =
+    !!process.env.VERCEL || process.env.NITRO_PRESET === "vercel"
+  if (!backend) {
+    if (onVercel) {
+      throw new Error(
+        "LANGGRAPH_BACKEND_URL is required for Vercel builds (set it per Vercel " +
+          "project: dev project -> dev langgraph URL, prod project -> prod langgraph URL).",
+      )
+    }
+    // Off Vercel with no backend configured (plain local `vite build`, tests): omit the
+    // proxy. Local dev gets its API via the E2E_HARNESS mock proxy above instead.
+    return undefined
+  }
+  return { "/dashboard/api/**": { proxy: `${backend}/dashboard/api/**` } }
+}
+
 const config = defineConfig({
   optimizeDeps: { include: ["workbox-window"] },
   worker: { format: "es" },
   plugins: [
     mockHarnessProxy(),
     devtools(),
-    nitro(),
+    nitro({ routeRules: backendProxyRouteRules() }),
     viteTsConfigPaths({
       projects: ["./tsconfig.json"],
     }),