Add NixOS module and improve UI navigation and rendering

The flake now includes a NixOS module that enables integration with
system configuration. The TUI has been enhanced with better file
switching using arrow keys, space bar support for toggling values,
improved scrolling logic, styled category headings, and visual selection
feedback. Documentation comments were streamlined and placeholder files
removed.

src/main.rs

@@ -12,6 +12,11 @@ use std::{
 
 fn main() -> Result<()> {
     let nix_modules: NixModules = get_modules();
+
+    for module in &nix_modules.system_modules {
+        println!("{} - ({})", module.category, module.path);
+    }
+
     let mut terminal: Terminal<CrosstermBackend<Stdout>> = ratatui::init();
     let mut app: App = App {
         exit: false,