[TOOL-3483] Dashboard: Fix Nebula subdomain rewrite for Vercel preview links (#6303)

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## PR-Codex overview
This PR updates the middleware logic to handle subdomains more flexibly, accommodating both the standard `nebula` subdomain and a new format used in Vercel previews.

### Detailed summary
- Changed the condition to check for `subdomain` being either `nebula` or starting with `nebula---`.
- Updated the path rewriting logic to prepend `nebula-app` to the existing paths when the condition is met.

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Author: MananTank

apps/dashboard/src/middleware.ts

@@ -33,7 +33,11 @@ export async function middleware(request: NextRequest) {
   const paths = pathname.slice(1).split("/");
 
   // nebula.thirdweb.com -> render page at app/nebula-app
-  if (subdomain === "nebula" && host) {
+  // on vercel preview, the format is nebula---thirdweb-www-git-<branch-name>.thirdweb-preview.com
+  if (
+    subdomain &&
+    (subdomain === "nebula" || subdomain.startsWith("nebula---"))
+  ) {
     const newPaths = ["nebula-app", ...paths];
     return rewrite(request, `/${newPaths.join("/")}`, undefined);
   }