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/hyprland/scripts/terminal.nix

@@ -1,9 +0,0 @@
-{pkgs, ...}: let
-  hakase-terminal-launch =
-    pkgs.writeShellScriptBin "hakase-terminal-launch" ''
-    '';
-in {
-  home.packages = [
-    hakase-terminal-launch
-  ];
-}