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/home/music.nix

@@ -24,8 +24,8 @@
   '';
 in {
   imports = [
-    ../../apps/jellyfin-tui/home.nix
-    ../../apps/cava/home.nix
+    ../../apps/jellyfin-tui
+    ../../apps/cava
   ];
   home.packages = [
     launch-music