Update artifact upload action to v3 for Gitea compatibility

.gitea/workflows/ci.yml

@@ -51,7 +51,8 @@ jobs:
         run: npx web-ext build -s . -a dist --overwrite-dest
         
       - name: Upload build artifact
-        uses: actions/upload-artifact@v4
+        # v4+ not supported on Gitea Actions/Forgejo, use v3 for compatibility
+        uses: actions/upload-artifact@v3
         with:
           name: web-ext-build
           path: dist/