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/ghostty/default.nix

@@ -0,0 +1,27 @@
+{
+  config,
+  pkgs,
+  ...
+}: {
+  home.file.".config/ghostty/screensaver".text = ''
+    window-padding-x = 0
+    window-padding-y = 0
+    window-padding-color = "extend-always"
+  '';
+  programs.ghostty = {
+    enable = true;
+    package =
+      if pkgs.stdenv.isDarwin
+      then pkgs.ghostty-bin
+      else pkgs.ghostty;
+    enableFishIntegration = true;
+
+    settings = {
+      confirm-close-surface = false;
+      app-notifications = false;
+      window-padding-x = 15;
+      window-padding-y = 15;
+      window-padding-balance = true;
+    };
+  };
+}