feat(editor): center on terrain (Home key), navigation improvements

- "Center on Terrain" (Home key or View menu): resets camera to center
  of loaded tile at 300 units altitude with 45-degree downward pitch.
  Essential for recovering when camera gets lost in empty space.
- Toast confirmation on center action

tools/editor/editor_app.hpp

@@ -73,6 +73,7 @@ public:
     void setSkyPreset(int preset); // 0=day, 1=dusk, 2=night
     void snapSelectedToGround();
     void flyToSelected();
+    void centerOnTerrain();
 
     // Multi-tile support
     void addAdjacentTile(int offsetX, int offsetY);