refactor: standardize config structure and consolidate modules

- Standardized apps/ directory by renaming all entry points (e.g., home.nix) to default.nix and updating imports to use directory paths.
- Consolidated system/ logic into modules/system/, eliminating the top-level system/ directory and redundant wrappers.
- Merged subsidiary utility scripts (e.g., hakase-popup.nix, switch-wallpaper.nix) into their parent default.nix files for better cohesion.
- Cleaned up unused files and updated all module references to reflect the new structure.

modules/system/home-manager.nix

@@ -0,0 +1,18 @@
+{
+  specialArgs,
+  myConfig,
+  inputs,
+  ...
+}: {
+  nixpkgs.overlays = [inputs.nur.overlays.default];
+  home-manager = {
+    backupFileExtension = "backup";
+    useGlobalPkgs = true;
+    useUserPackages = true;
+    extraSpecialArgs = specialArgs;
+    users.${myConfig.nixos.username} = import ../home/hakase.nix;
+  };
+
+  imports = [
+  ];
+}