Fix a river in Balkans not connecting to the sea (#4249)

Resolves #4248

## Description:

Fix river not connected in Balkans map along the map border. The map
generator accidentally deleted some columns of pixels along the map
limits, and it disconnected a river.

<img width="588" height="482" alt="image"
src="https://github.com/user-attachments/assets/2c78b6bd-d669-4aef-bc1d-c69d4aeed162"
/>

Updated version

<img width="290" height="311" alt="image"
src="https://github.com/user-attachments/assets/f315bdfc-bcca-400d-95a7-876c14e47400"
/>

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory

## Please put your Discord username so you can be contacted if a bug or
regression is found:

tri.star1011

resources/maps/balkans/manifest.json

@@ -200,17 +200,17 @@
   "id": "Balkans",
   "map": {
     "height": 2048,
-    "num_land_tiles": 2478822,
+    "num_land_tiles": 2478805,
     "width": 2048
   },
   "map16x": {
     "height": 512,
-    "num_land_tiles": 148761,
+    "num_land_tiles": 148759,
     "width": 512
   },
   "map4x": {
     "height": 1024,
-    "num_land_tiles": 610636,
+    "num_land_tiles": 610629,
     "width": 1024
   },
   "multiplayer_frequency": 6,