fix: downgrade upload/download-artifact to v3 for Gitea Actions compatibility

actions/upload-artifact@v4 uses a new artifact API that requires
specific runner-side configuration not present in all act_runner setups.
v3 uses the simpler API that works reliably with self-hosted Gitea runners.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/docker-build.yml

@@ -103,7 +103,7 @@ jobs:
         working-directory: desktop
         run: npx electron-builder --win nsis --x64 --publish never
 
-      - uses: actions/upload-artifact@v4
+      - uses: actions/upload-artifact@v3
         with:
           name: desktop-linux-windows-${{ github.sha }}
           path: |
@@ -141,7 +141,7 @@ jobs:
     if: startsWith(github.ref, 'refs/tags/v')
     steps:
       - name: Download desktop artifacts
-        uses: actions/download-artifact@v4
+        uses: actions/download-artifact@v3
         with:
           name: desktop-linux-windows-${{ github.sha }}
           path: ./release-assets