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/backlight.nix

@@ -0,0 +1,12 @@
+{
+  pkgs,
+  myConfig,
+  config,
+  ...
+}: {
+  hardware.i2c.enable = true;
+  # boot.extraModulePackages = [config.boot.kernelPackages.ddcci-driver];
+  # boot.kernelModules = ["ddcci_backlight"];
+  environment.systemPackages = with pkgs; [ddcutil];
+  users.users.${myConfig.nixos.username}.extraGroups = ["i2c"];
+}