chore: enable release build

Author: kkweon

.github/workflows/ci.yml

@@ -84,8 +84,8 @@ jobs:
         with:
           channel: "stable"
       - run: flutter pub get
-      - run: flutter build web
+      - run: flutter build web --release
       - uses: JamesIves/github-pages-deploy-action@4.1.4
         with:
           branch: gh-pages
-          folder: build/web
+          folder: build/web