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/fonts.nix

@@ -0,0 +1,21 @@
+{pkgs, ...}: {
+  fonts.packages = with pkgs; [
+    nerd-fonts.fira-code
+    nerd-fonts.jetbrains-mono
+
+    # google
+    noto-fonts
+    noto-fonts-cjk-sans
+    noto-fonts-cjk-serif
+    noto-fonts-color-emoji
+    roboto
+    open-sans
+    lato
+    montserrat
+    inter
+    poppins
+    fira-code
+    fira-code-symbols
+    jetbrains-mono
+  ];
+}