fix(www): reverse proxy /tanstack/* to v1 preview during alpha (#1812)

### Summary
This PR adds a reverse proxy rewrite in `apps/www/next.config.ts`
routing `/tanstack/:path*` requests to
`https://arkenv-v1.vercel.app/tanstack/:path*`.

### Why
The official TanStack CLI add-on for ArkEnv is deployed on the `v1`
branch at `https://arkenv-v1.vercel.app/tanstack/info.json`. Because
production docs (`https://arkenv.js.org`) are deployed from `main`
(which flows from `dev`), this rewrite allows
`https://arkenv.js.org/tanstack/info.json` to resolve immediately during
the alpha prerelease phase without URL rot or waiting for the full v1
merge.

Once merged to `dev`, this change can be synced directly to `main` via
`sync-main.yml` (Category 1 doc/infra sync).

**Blocks / Relates to:** #1811 (feat: add official TanStack CLI add-on
for ArkEnv)

---------

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
Co-authored-by: pullfrog[bot] <226033991+pullfrog[bot]@users.noreply.github.com>

Author: 3 people

apps/www/next.config.ts

@@ -79,6 +79,15 @@ const config = {
 				source: `${POSTHOG_PROXY_PREFIX}/:path*`,
 				destination: `${POSTHOG_API_ENDPOINT}/:path*`,
 			},
+			/**
+			 * Proxy TanStack CLI add-on to v1 preview during alpha.
+			 * Keeps v1 as the single source of truth for template updates.
+			 * TODO(v1-ga): Remove this rewrite once v1 is merged into dev/main.
+			 */
+			{
+				source: "/tanstack/:path*",
+				destination: "https://arkenv-v1.vercel.app/tanstack/:path*",
+			},
 		];
 	},
 	// This is required to support PostHog trailing slash API requests