🚀 Major refactor: Reorganize project structure

- Move WebAPI/ → src/Backend/DiunaBI.WebAPI/
- Move Frontend/ → src/Frontend/
- Move Deployment/ → deploy/
- Add proper .NET 8 solution structure
- Add plugin architecture with DiunaBI.Plugins.Morska
- Clean

global.json

@@ -1,7 +0,0 @@
-{
-  "sdk": {
-    "version": "9.0.0",
-    "rollForward": "latestMajor",
-    "allowPrerelease": false
-  }
-}