fix: update GitHub Actions artifact actions to v4

Resolves deprecation warnings by updating:
- actions/upload-artifact@v3 → v4
- actions/download-artifact@v3 → v4

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: vtemian

.github/workflows/ci.yml

@@ -97,7 +97,7 @@ jobs:
       run: uv build
 
     - name: Upload build artifacts
-      uses: actions/upload-artifact@v3
+      uses: actions/upload-artifact@v4
       with:
         name: dist
         path: dist/
@@ -137,7 +137,7 @@ jobs:
       id-token: write
 
     steps:
-    - uses: actions/download-artifact@v3
+    - uses: actions/download-artifact@v4
       with:
         name: dist
         path: dist/