Add clap dependency and update app for CLI integration

Update Cargo files to add clap and related dependencies, restructure App
to handle configuration from CLI, and update nix module to support
config source tracking. Add new cli module and adjust main to initialize
modules at startup.

Cargo.toml

@@ -1,9 +1,10 @@
 [package]
-name = "garandos-tui"
+name = "garandos_tui"
 version = "0.1.0"
 edition = "2024"
 
 [dependencies]
+clap = { version = "4.5.53", features = ["derive"] }
 crossterm = "0.29.0"
 ratatui = "0.29.0"
 rnix = "0.12.0"