fix: Enable Corepack to install Yarn in CI workflow

- Add corepack enable step before yarn commands
- Corepack comes with Node.js and manages package managers
- Fixes 'Unable to locate executable file: yarn' error
- Required for Yarn 4 (Berry) specified in package.json

.gitea/workflows/ci.yml

@@ -134,6 +134,8 @@ jobs:
           node-version: '24'
           cache: 'yarn'
           cache-dependency-path: bugulma/frontend/yarn.lock
+      - name: Enable Corepack (for Yarn)
+        run: corepack enable
       - name: Install dependencies
         working-directory: bugulma/frontend
         run: yarn install --frozen-lockfile