feat: Implement user management with a new add_user binary and refactor state handling into a dedicated store module.

backend/src/bin/add_user.rs

@@ -0,0 +1,55 @@
+use backend::items::Person;
+use backend::store::{self, User, save_state};
+use std::io::{self, Write};
+
+fn main() {
+    println!("Add User Tool");
+    println!("-------------");
+
+    print!("Username: ");
+    io::stdout().flush().unwrap();
+    let mut username = String::new();
+    io::stdin().read_line(&mut username).unwrap();
+    let username = username.trim().to_string();
+
+    if username.is_empty() {
+        println!("Username cannot be empty.");
+        return;
+    }
+
+    print!("Password: ");
+    io::stdout().flush().unwrap();
+    let mut password = String::new();
+    io::stdin().read_line(&mut password).unwrap();
+    let password = password.trim().to_string();
+
+    if password.is_empty() {
+        println!("Password cannot be empty.");
+        return;
+    }
+
+    let (games, mut users) = store::load_state().unwrap_or_else(|| {
+        println!("No existing state found. Creating new state.");
+        (Vec::new(), Vec::new())
+    });
+
+    if users.iter().any(|u| u.person.name == username) {
+        println!("User '{}' already exists.", username);
+        return;
+    }
+
+    let password_hash = bcrypt::hash(&password, bcrypt::DEFAULT_COST).unwrap();
+
+    let new_user = User {
+        person: Person {
+            name: username.clone(),
+            opinion: Vec::new(),
+        },
+        password_hash,
+    };
+
+    users.push(new_user);
+    save_state(&games, &users);
+
+    println!("User '{}' added successfully.", username);
+}