Refactor: reorganize scripts with domain-driven design structure

Move all C# scripts from flat structure to organized folders:
- Core/ for game logic (Game, GameState, GameSettings)
- UI/Menus/ and UI/Dialogs/ for user interface
- Systems/ for reusable systems (Save, Localization)
- Data/ for data models and configuration
- Added framework for future: Gameplay/, Story/, Modding/

Update all .tscn scene files to reference new script paths.
Fix timing issue in AdvancedSaveDialog focus handling.

scripts/Gameplay/Units/BaseUnit.cs

@@ -0,0 +1,9 @@
+// Placeholder för unit system
+// Detta kommer att innehålla bas-klasser för enheter i spelet
+
+namespace TheGame.Gameplay.Units
+{
+    // TODO: Implementera BaseUnit class
+    // TODO: Implementera Unit stats system
+    // TODO: Implementera Unit behaviors
+}

scripts/Gameplay/Units/BaseUnit.cs.uid

@@ -0,0 +1 @@
+uid://bdm5xfw58nd5i