refactor: rename namespace to FiscalFlow and upgrade to .NET 10

- Rename InvoiceMaster.* to FiscalFlow.* namespace
- Upgrade from .NET 8 to .NET 10
- Update all NuGet packages to latest versions
- Update C# language version to 14.0

backend/global.json

@@ -1,6 +1,6 @@
 {
   "sdk": {
-    "version": "8.0.100",
+    "version": "10.0.100",
     "rollForward": "latestFeature"
   }
 }