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/package.json

@@ -5,7 +5,6 @@
   "scripts": {
     "dev": "vite dev --port 3000",
     "build": "vite build",
-    "vercel-build": "vite build && node scripts/build-vercel-output.mjs",
     "preview": "vite preview",
     "test": "vitest run",
     "lint": "eslint",

ui/scripts/build-vercel-output.mjs

@@ -1,47 +0,0 @@
-// Repackage the static Vite/TanStack-Start build (.output/public) into the Vercel
-// Build Output API layout (.vercel/output), generating the /dashboard/api/* proxy
-// destination from a per-project env var.
-//
-// Why this exists: prod deploys via a fast-forward-only promotion, so the `dev` and
-// `main` branches are byte-identical and a committed vercel.json CANNOT differ between
-// them. The app is same-origin only (no backend CORS), so the per-environment backend
-// must be the PROXY DESTINATION, supplied per Vercel project as LANGGRAPH_BACKEND_URL.
-// Vercel reads vercel.json BEFORE the build and never interpolates env into it, so the
-// routes are emitted here (read AFTER the build) where process.env is available.
-import { cp, mkdir, writeFile, rm } from "node:fs/promises"
-
-const backend = process.env.LANGGRAPH_BACKEND_URL
-if (!backend) {
-  throw new Error(
-    "LANGGRAPH_BACKEND_URL is required at build time (set it per Vercel project: " +
-      "dev project -> dev langgraph URL, prod project -> prod langgraph URL).",
-  )
-}
-const base = backend.replace(/\/$/, "")
-
-await rm(".vercel/output", { recursive: true, force: true })
-await mkdir(".vercel/output/static", { recursive: true })
-await cp(".output/public", ".vercel/output/static", { recursive: true })
-
-await writeFile(
-  ".vercel/output/config.json",
-  JSON.stringify(
-    {
-      version: 3,
-      routes: [
-        // Same-origin proxy to this project's backend (proxy, not redirect, so the
-        // osw_session cookie stays first-party on the Vercel origin).
-        { src: "^/dashboard/api/(.*)$", dest: `${base}/dashboard/api/$1` },
-        // Serve hashed assets, the PWA manifest/service worker, and _shell.html itself
-        // BEFORE the SPA catch-all, or static files get swallowed by the fallback.
-        { handle: "filesystem" },
-        // SPA fallback (replaces the old vercel.json /(.*) -> /_shell.html rewrite).
-        { src: "^/.*$", dest: "/_shell.html" },
-      ],
-    },
-    null,
-    2,
-  ) + "\n",
-)
-
-console.log(`[build-vercel-output] wrote .vercel/output/config.json (backend: ${base})`)

ui/vercel.json

@@ -1,4 +1,5 @@
 {
+  "$schema": "https://openapi.vercel.sh/vercel.json",
   "framework": null,
-  "buildCommand": "bun run vercel-build"
+  "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"],
     }),