add rivers to britannia (#2026)

## Description:

Adds rivers to the Britannia map. This uses OpenStreetMap river layer
data. Therefore the credits to this should be added somewhere visible
according to
https://osmfoundation.org/w/index.php?title=Licence/Attribution_Guidelines#Computer_games_and_simulations

See the issue https://github.com/openfrontio/OpenFrontIO/issues/2023

## 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 (N/A)
- [X] I have added relevant tests to the test directory (N/A)
- [X] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

## Please put your Discord username so you can be contacted if a bug or
regression is found:
@dxtron_28992 (application to join Dev discord still pending)

<img width="1335" height="932" alt="image"
src="https://github.com/user-attachments/assets/158bc616-1618-4117-9a21-9b04cb70a949"
/>

---------

Co-authored-by: Vivacious Box <jon@rouillard.org>

resources/maps/britannia/manifest.json

@@ -1,17 +1,17 @@
 {
   "map": {
     "height": 1396,
-    "num_land_tiles": 938800,
+    "num_land_tiles": 933860,
     "width": 2000
   },
   "map16x": {
     "height": 349,
-    "num_land_tiles": 56046,
+    "num_land_tiles": 55046,
     "width": 500
   },
   "map4x": {
     "height": 698,
-    "num_land_tiles": 231151,
+    "num_land_tiles": 228988,
     "width": 1000
   },
   "name": "Britannia",