feat(ui): derive Vercel proxy backend from per-project env var (#75)

Generate the /dashboard/api/* proxy destination at build time from
LANGGRAPH_BACKEND_URL via the Vercel Build Output API, instead of hardcoding
it in ui/vercel.json. This lets the dev and main branches stay byte-identical
(required by the fast-forward-only prod promotion) while each Vercel project
proxies to its own LangGraph backend purely via its own env var — so both the
dev and prod dashboards can be git-linked.

- ui/vercel.json: drop the hardcoded rewrites; buildCommand -> bun run vercel-build
- ui/package.json: add vercel-build = vite build && node scripts/build-vercel-output.mjs
- ui/scripts/build-vercel-output.mjs: emit .vercel/output/config.json with
  [ proxy /dashboard/api/* -> $LANGGRAPH_BACKEND_URL, filesystem, SPA fallback ];
  throws if LANGGRAPH_BACKEND_URL is unset so a misconfig fails the build.

Keeps the app same-origin (no CORS/auth change); preserves the _shell.html SPA
fallback. Set LANGGRAPH_BACKEND_URL per Vercel project (All Environments).

ui/package.json

@@ -5,6 +5,7 @@
   "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

@@ -0,0 +1,47 @@
+// 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

@@ -1,15 +1,4 @@
 {
   "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 vercel-build"
 }