chore: update browser, theme, and locale configurations

- Replace Librewolf with Firefox as the backup browser
- Update default and extra locales to en_US.UTF-8 and de_DE.UTF-8 respectively
- Change theme fetch setting from 'none' to 'fastfetch' and update wallpaper
- Minor formatting cleanup in THEMES.md

hosts/laptop/variables.nix

@@ -16,8 +16,8 @@
     keyboardLayout = "de";
 
     timeZone = "Europe/Berlin";
-    defaultLocale = "de_DE.UTF-8";
-    extraLocale = "en_US.UTF-8";
+    defaultLocale = "en_US.UTF-8";
+    extraLocale = "de_DE.UTF-8";
 
     git = {
       username = "kptltd00m";