test(smoke): add astro build verification for all demos and templates (#361)

Smoke tests only ran astro dev, which uses Vite's on-demand compilation
and never invokes the adapter build pipeline. Build failures in
cloudflare/playground demos (or any adapter-dependent site) passed CI
undetected. Add a build verification suite that runs astro build for
every site in the matrix before the existing runtime tests.

Bumps smoke test CI timeout from 15 to 30 minutes to accommodate.

.github/workflows/ci.yml

@@ -118,7 +118,7 @@ jobs:
   test-smoke:
     name: Smoke Tests
     runs-on: ubuntu-latest
-    timeout-minutes: 15
+    timeout-minutes: 30
     services:
       postgres:
         image: postgres:17