Bump actions/checkout v4 → v6

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

.forgejo/workflows/build.yml

@@ -28,7 +28,7 @@ jobs:
     container:
       image: ghcr.io/cirruslabs/flutter:stable
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Install dependencies
         run: flutter pub get
@@ -46,7 +46,7 @@ jobs:
           fi
 
       - name: Upload artifact
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: ${{ steps.artifact.outputs.name }}
           path: build/app/outputs/flutter-apk/app-release.apk

.forgejo/workflows/ci.yml

@@ -25,7 +25,7 @@ jobs:
     container:
       image: ghcr.io/cirruslabs/flutter:stable
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Install dependencies
         run: flutter pub get