Fix broken flag images (#1078)

## Description:

Some flag images are missing from the flag menu, and can thus not be
used as a flag by the player in the game.

This this fixes that by equating the names in countries.json to the
filenames of the flags.

Example of missing flag image from before this fix:


![afbeelding](https://github.com/user-attachments/assets/ec7aecdc-ee69-4852-98f9-14f3dc05febc)


## 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
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [x] I understand that submitting code with bugs that could have been
caught through manual testing blocks releases and new features for all
contributors

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

tryout33

src/client/data/countries.json

@@ -473,7 +473,7 @@
     "name": "Comoros"
   },
   {
-    "code": "communist flag",
+    "code": "Communist flag",
     "name": "Communist Flag"
   },
   {
@@ -740,7 +740,7 @@
     "name": "Franks"
   },
   {
-    "code": "french foreign legion",
+    "code": "French foreign legion",
     "name": "French Foreign Legion"
   },
   {
@@ -1188,7 +1188,7 @@
     "name": "Luxembourg"
   },
   {
-    "code": "lydia",
+    "code": "Lydia",
     "continent": "Asia",
     "name": "Lydia"
   },
@@ -1238,7 +1238,7 @@
     "name": "Malta"
   },
   {
-    "code": "Māori Flag",
+    "code": "Māori flag",
     "continent": "Oceania",
     "name": "Māori Flag"
   },
@@ -1466,7 +1466,7 @@
     "name": "Normandy"
   },
   {
-    "code": "North Karelia",
+    "code": "North karelia",
     "continent": "Europe",
     "name": "North Karelia"
   },