fix: Move checkout step before artifact downloads in R2 upload job

The actions/checkout@v4 step was running after downloading artifacts,
causing it to wipe out the downloaded files. This resulted in empty
build directories and missing artifacts in the index.html.

By moving checkout to the beginning of the upload-to-r2 job, artifacts
are downloaded after the repository is cloned, preserving all build
files for proper R2 upload.

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

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

.github/workflows/build-multiplatform.yml

@@ -456,6 +456,11 @@ jobs:
     if: always() && (needs.build-android.result == 'success' || needs.build-ios.result == 'success' || needs.build-macos.result == 'success' || needs.build-windows.result == 'success')
 
     steps:
+      - name: Checkout code for git log
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 10  # Fetch last 10 commits
+
       - name: Download Android APK
         if: needs.build-android.result == 'success'
         uses: actions/download-artifact@v4
@@ -505,11 +510,6 @@ jobs:
             echo "This is expected if no platform builds succeeded"
           fi
 
-      - name: Checkout code for git log
-        uses: actions/checkout@v4
-        with:
-          fetch-depth: 10  # Fetch last 10 commits
-
       - name: Generate build metadata
         id: metadata
         run: |