Change region to use top-left and bottom-right coordinates

Replace center+delta region representation with a bounding box defined
by topLeft and bottomRight coordinate pairs, which maps more directly
to the visible map area.

https://claude.ai/code/session_0162emnCi1KxW5FkTNn2ZrvH

src/entities/SharedEntities.ts

@@ -12,9 +12,7 @@ export interface ICoordinates {
 }
 
 export interface IRegion {
-  latitude: number;
-  longitude: number;
-  latitudeDelta: number;
-  longitudeDelta: number;
+  topLeft: ICoordinates;
+  bottomRight: ICoordinates;
 }