CI: create build/ before compiling the server; drop artifact upload

upload-artifact@v4 is unsupported on Gitea (GHES error) and failed the
release job after a successful build; the release asset is the
artifact. dart compile exe doesn't create the output directory.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -32,7 +32,7 @@ jobs:
       - run: dart test
         working-directory: packages/server
       # Compiling the dedicated server proves it stays Flutter-free.
-      - run: dart compile exe bin/server.dart -o build/watch-together-server
+      - run: mkdir -p build && dart compile exe bin/server.dart -o build/watch-together-server
         working-directory: packages/server
       - run: build/watch-together-server --help
         working-directory: packages/server