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/error.rs

@@ -0,0 +1,28 @@
+use axum::http::StatusCode;
+use axum::response::{IntoResponse, Response};
+use serde_json::json;
+
+pub enum AppError {
+    Unauthorized,
+    BadRequest(String),
+    Internal(String),
+}
+
+impl IntoResponse for AppError {
+    fn into_response(self) -> Response {
+        let (status, message) = match &self {
+            Self::Unauthorized => (StatusCode::UNAUTHORIZED, "Unauthorized".to_owned()),
+            Self::BadRequest(msg) => (StatusCode::BAD_REQUEST, msg.clone()),
+            Self::Internal(msg) => (StatusCode::INTERNAL_SERVER_ERROR, msg.clone()),
+        };
+
+        let body = axum::Json(json!({ "error": message }));
+        (status, body).into_response()
+    }
+}
+
+impl From<std::io::Error> for AppError {
+    fn from(err: std::io::Error) -> Self {
+        Self::Internal(err.to_string())
+    }
+}