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/services.nix

@@ -0,0 +1,41 @@
+{
+  pkgs,
+  myConfig,
+  ...
+}: {
+  services = {
+    greetd = {
+      enable = true;
+      settings = {
+        initial_session = {
+          command = "${pkgs.hyprland}/bin/Hyprland";
+          user = "${myConfig.nixos.username}";
+        };
+        default_session = {
+          command = "${pkgs.tuigreet}/bin/tuigreet --time --remember --cmd Hyprland";
+        };
+      };
+    };
+
+    # essentials
+    openssh.enable = true; # FIXME: remove when done
+    blueman.enable = true;
+
+    # for encryption support for unfree apps
+    gnome.gnome-keyring.enable = true;
+
+    # Complements printer support
+    printing.enable = true;
+    avahi = {
+      enable = true;
+      nssmdns4 = true;
+      openFirewall = true;
+    };
+    pipewire = {
+      enable = true;
+      alsa = {
+        enable = true;
+      };
+    };
+  };
+}