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/boot.nix

@@ -0,0 +1,29 @@
+{pkgs, ...}: {
+  boot = {
+    consoleLogLevel = 3;
+    initrd.verbose = false;
+    loader = {
+      timeout = 0;
+      systemd-boot = {
+        enable = true;
+      };
+
+      efi = {
+        canTouchEfiVariables = true;
+        efiSysMountPoint = "/boot";
+      };
+    };
+    initrd = {
+      systemd.enable = true;
+    };
+    plymouth = {
+      enable = true;
+      # themePackages = [
+      #   (pkgs.adi1090x-plymouth-themes.override {
+      #     selected_themes = ["circle_hud"];
+      #   })
+      # ];
+      # theme = "circle_hud";
+    };
+  };
+}