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

schema.graphqls

@@ -18,10 +18,8 @@ type System {
 }
 
 type Region {
-  latitude: Float!
-  longitude: Float!
-  latitudeDelta: Float!
-  longitudeDelta: Float!
+  topLeft: Coordinates!
+  bottomRight: Coordinates!
 }
 
 type ParkingSystem {