Add configuration loading and error handling

Introduces a new `config` module for loading application settings from
environment variables and a `users.toml` file. An `error` module is
added to handle application-specific errors gracefully. An
`.env.example` file is created to show available configuration options,
and `users.toml.example` provides a template for user data. The main
function now initializes the configuration and prints the server port.

server/src/main.rs

@@ -1,3 +1,12 @@
-fn main() {
-    println!("Hello, world!");
+mod config;
+mod error;
+
+use config::Config;
+use std::sync::Arc;
+
+#[tokio::main]
+async fn main() {
+    dotenvy::dotenv().ok();
+    let config = Arc::new(Config::from_env());
+    println!("Server starting on port {}", config.server_port);
 }