ci: upgrade actions/upload-artifact and download-artifact to v4

v3 is deprecated and GitHub now rejects it, causing build-webui to fail.

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

.github/workflows/build.yml

@@ -161,7 +161,7 @@ jobs:
           cd webui && pnpm build
 
       - name: Upload artifact
-        uses: actions/upload-artifact@v3
+        uses: actions/upload-artifact@v4
         with:
           name: dist
           path: webui/dist
@@ -227,7 +227,7 @@ jobs:
           password: ${{ secrets.ACCESS_TOKEN }}
 
       - name: Download artifact
-        uses: actions/download-artifact@v3
+        uses: actions/download-artifact@v4
         with:
           name: dist
           path: backend/src/dist
@@ -282,7 +282,7 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Download artifact webui
-        uses: actions/download-artifact@v3
+        uses: actions/download-artifact@v4
         with:
           name: dist
           path: webui/dist
@@ -292,7 +292,7 @@ jobs:
           cd webui && ls -al && tree && zip -r dist.zip dist
 
       - name: Download artifact app
-        uses: actions/download-artifact@v3
+        uses: actions/download-artifact@v4
         with:
           name: dist
           path: backend/src/dist