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.

apps/git/default.nix

@@ -0,0 +1,10 @@
+{myConfig, ...}: {
+  programs.git = {
+    enable = true;
+    settings = {
+      user.name = "${myConfig.git.username}";
+      user.email = "${myConfig.git.email}";
+      init.defaultBranch = "${myConfig.git.defaultBranch}";
+    };
+  };
+}