misc(refactor): extract command handlers and centralize error handling

- Move command-specific logic out of main.rs into Server::handle_* methods
  (parsing remains in main and handlers accept already-parsed input).
- Handlers now return Result and return Err("ERROR:...") instead of sending
  error messages directly to clients.
- main.rs now logs handler errors to console and sends the error string to
  the client (removed `continue` statements after error sends).
- Move end_game_check from main.rs into lib.rs for reuse.
- Update handler implementations to follow the new error-returning pattern.

src/tournaments/mod.rs

@@ -2,16 +2,18 @@ use std::net::SocketAddr;
 
 use async_trait::async_trait;
 
-use crate::{*};
+use crate::*;
 
 pub mod round_robin;
 pub use round_robin::RoundRobin;
 
 #[async_trait]
 pub trait Tournament {
-	fn new(ready_players: &[SocketAddr]) -> Self where Self: Sized;
-	async fn next(&mut self, clients: &Clients, matches: &Matches, observers: &Observers);
-	async fn start(&mut self, clients: &Clients, matches: &Matches);
-	async fn cancel(&mut self, clients: &Clients, matches: &Matches, observers: &Observers);
-	fn is_completed(&self) -> bool;
-}
+    fn new(ready_players: &[SocketAddr]) -> Self
+    where
+        Self: Sized;
+    async fn next(&mut self, clients: &Clients, matches: &Matches, observers: &Observers);
+    async fn start(&mut self, clients: &Clients, matches: &Matches);
+    async fn cancel(&mut self, clients: &Clients, matches: &Matches, observers: &Observers);
+    fn is_completed(&self) -> bool;
+}