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/gen/users.sql.go

@@ -0,0 +1,66 @@
+// Code generated by sqlc. DO NOT EDIT.
+// versions:
+//   sqlc v1.30.0
+// source: users.sql
+
+package gen
+
+import (
+	"context"
+)
+
+const createUser = `-- name: CreateUser :one
+INSERT INTO users (id, username, password_hash)
+VALUES (?, ?, ?)
+RETURNING id, username, password_hash, created_at
+`
+
+type CreateUserParams struct {
+	ID           string
+	Username     string
+	PasswordHash string
+}
+
+func (q *Queries) CreateUser(ctx context.Context, arg CreateUserParams) (User, error) {
+	row := q.db.QueryRowContext(ctx, createUser, arg.ID, arg.Username, arg.PasswordHash)
+	var i User
+	err := row.Scan(
+		&i.ID,
+		&i.Username,
+		&i.PasswordHash,
+		&i.CreatedAt,
+	)
+	return i, err
+}
+
+const getUserByID = `-- name: GetUserByID :one
+SELECT id, username, password_hash, created_at FROM users WHERE id = ?
+`
+
+func (q *Queries) GetUserByID(ctx context.Context, id string) (User, error) {
+	row := q.db.QueryRowContext(ctx, getUserByID, id)
+	var i User
+	err := row.Scan(
+		&i.ID,
+		&i.Username,
+		&i.PasswordHash,
+		&i.CreatedAt,
+	)
+	return i, err
+}
+
+const getUserByUsername = `-- name: GetUserByUsername :one
+SELECT id, username, password_hash, created_at FROM users WHERE username = ?
+`
+
+func (q *Queries) GetUserByUsername(ctx context.Context, username string) (User, error) {
+	row := q.db.QueryRowContext(ctx, getUserByUsername, username)
+	var i User
+	err := row.Scan(
+		&i.ID,
+		&i.Username,
+		&i.PasswordHash,
+		&i.CreatedAt,
+	)
+	return i, err
+}