Add subcommand support with colored help banner

Enable `clap` color feature, introduce subcommands (Sync, Add, Rm, Ls)
and a
custom before‑help banner, and update `main` to return a `Result`.

src/main.rs

@@ -1,7 +1,8 @@
+use std::io::Result;
 use veil_rs::cli::Cli;
 
-fn main() {
+fn main() -> Result<()> {
     let args: Cli = Cli::get_args();
 
-    println!("Hello, world!");
+    Ok(())
 }