fix(ci): use sparse checkout to avoid large data/ directory

The repository has 314K+ files including backup data that exceeds
the CI runner's disk space. This change uses sparse checkout to only
fetch frontend/ and schemas/ directories needed for the build.

.forgejo/workflows/deploy-frontend.yml

@@ -36,8 +36,13 @@ jobs:
     runs-on: ubuntu-latest
     
     steps:
-      - name: Checkout repository
+      - name: Checkout repository (sparse)
         uses: actions/checkout@v4
+        with:
+          sparse-checkout: |
+            frontend
+            schemas
+          sparse-checkout-cone-mode: false
 
       - name: Setup Node.js
         uses: actions/setup-node@v4