fix: add output field to playground build task so Vercel cache restores dist

The playground was missed in 0443605 which added output: ["dist/**"] to
all library package build tasks. Without it, vp replays terminal output on
cache hits but never restores the actual dist/ files, causing Vercel to
fail with "No Output Directory named dist found".

Author: nperez0111

playground/vite.config.ts

@@ -66,6 +66,7 @@ export default defineConfig(((conf: { command: string }) => ({
           { auto: true },
           { pattern: "!**/*.tsbuildinfo", base: "workspace" },
         ],
+        output: ["dist/**"],
       },
     },
   },