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/fish/default.nix

@@ -0,0 +1,19 @@
+{
+  pkgs,
+  myConfig,
+  ...
+}: {
+  programs.fish = {
+    enable = true;
+    interactiveShellInit = ''
+      set fish_greeting
+      fish_vi_key_bindings
+      zoxide init fish | source
+      zoxide init fish --cmd cd | source
+    '';
+    shellAliases = myConfig.terminal.aliases;
+
+    plugins = with pkgs.fishPlugins; [
+    ];
+  };
+}