📦 Created config files with common code

- Created common.nix for various configs
- Created theme.nix for theme related configs
- Moved some code to more logical files
- Moved some standalone files into nix multiline strings, in order to
  inject data

home-manager/default.nix

@@ -4,7 +4,8 @@
   ...
 }:
 let
-  username = "martin";
+  common = import ../common.nix;
+  username = common.username;
   homeDir = "/home/${username}";
 in
 {
@@ -31,7 +32,7 @@ in
     # You can update Home Manager without changing this value. See
     # the Home Manager release notes for a list of state version
     # changes in each release.
-    stateVersion = "24.11";
+    stateVersion = common.system.version;
   };
 
   programs = {