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/hardware.nix

@@ -0,0 +1,23 @@
+{
+  pkgs,
+  myConfig,
+  ...
+}: {
+  hardware = {
+    bluetooth = {
+      enable = true;
+      powerOnBoot = true;
+    };
+  };
+  networking = {
+    hostName = myConfig.nixos.hostname;
+    networkmanager.enable = true;
+    networkmanager.wifi.backend = "iwd";
+    wireless.iwd.enable = true;
+    firewall = {
+      enable = true;
+      allowedTCPPorts = [22 80];
+      allowedUDPPorts = [53];
+    };
+  };
+}