fix(builder): pin Vercel deploys to nitro stable, not nitro-nightly

Verified end-to-end with a real TanStack Start app deployed to Vercel:

  - 'npm:nitro-nightly@latest' (today: 3.0.1-20260508-123613-d2e64906)
    -> deploys, but every request returns 508 INFINITE_LOOP_DETECTED
  - 'latest' (today: nitro@3.0.260429-beta)
    -> deploys and serves 200 OK

The 508 loop is an upstream Nitro/Vercel preset bug fixed in the stable
beta stream around 3.0.260311-beta. Nightly hasn't picked the fix up yet
as of the test date, so the deploy button would otherwise produce a
broken site for every user who clicks it.

Inline doc explains the deviation from the Railway path (which still
uses nitro-nightly) so future contributors don't 'normalize' it back.

Author: HiMarioLopez

src/utils/provider-config.server.ts

@@ -122,13 +122,18 @@ function getRailwayConfig(): ProviderConfigResult {
  * no extra config files are required. Deployments run on Vercel Functions
  * with Fluid Compute by default.
  *
+ * Uses the stable `nitro` package (currently a beta on `latest`) rather
+ * than `nitro-nightly`. As of nightly 3.0.1-20260508-*, the Vercel preset
+ * hits a 508 INFINITE_LOOP_DETECTED on the first request; the fix lives
+ * in the stable beta stream (3.0.260311-beta and later).
+ *
  * @see https://vercel.com/docs/frameworks/full-stack/tanstack-start
  */
 function getVercelConfig(): ProviderConfigResult {
   return {
     files: {},
     devDependencies: {
-      nitro: 'npm:nitro-nightly@latest',
+      nitro: 'latest',
     },
   }
 }