Add user authentication and game persistence with SQLite

- User registration/login with bcrypt password hashing
- SQLite database with goose migrations and sqlc-generated queries
- Games and players persisted to database, resumable after restart
- Guest play still supported alongside authenticated users
- Auth UI components (login/register forms, auth header, guest banner)

db/queries/users.sql

@@ -0,0 +1,10 @@
+-- name: CreateUser :one
+INSERT INTO users (id, username, password_hash)
+VALUES (?, ?, ?)
+RETURNING *;
+
+-- name: GetUserByID :one
+SELECT * FROM users WHERE id = ?;
+
+-- name: GetUserByUsername :one
+SELECT * FROM users WHERE username = ?;