fix: use macos-15 and stable NuGet source

Author: mateof

.github/workflows/buildrelease.yml

@@ -299,7 +299,7 @@ jobs:
   # ==========================================
   build-macos:
     name: Build macOS App
-    runs-on: macos-13
+    runs-on: macos-15
     steps:
       - name: '📄 Checkout'
         uses: actions/checkout@v4
@@ -324,7 +324,8 @@ jobs:
 
       - name: '🍎 Install MAUI workload'
         run: |
-          dotnet workload install maui-maccatalyst --skip-sign-check
+          # Install workload using only stable sources
+          dotnet workload install maui-maccatalyst --skip-sign-check --source https://api.nuget.org/v3/index.json
           dotnet workload list
           # Show what SDK packs were installed
           ls -la ~/.dotnet/packs/ | grep -i catalyst || true