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/user.nix

@@ -0,0 +1,17 @@
+{
+  pkgs,
+  myConfig,
+  config,
+  ...
+}: {
+  users = {
+    users = {
+      ${myConfig.nixos.username} = {
+        isNormalUser = true;
+        extraGroups = ["wheel" "networkmanager" "input" "video"];
+        hashedPasswordFile = config.sops.secrets.default_password.path; # FIXME: may not work!
+        useDefaultShell = true;
+      };
+    };
+  };
+}