Implement Client struct with functional options

Add client.go with:
- Client struct with baseURL, username, remoteKey, token, tokenExp,
  httpClient, retryConf, logger, and sync.RWMutex for thread safety
- NewClient constructor accepting username, remoteKey, and variadic options
- Default values: 30s timeout, Checkvist base URL

Add options.go with:
- RetryConfig struct (MaxRetries, BaseDelay, MaxDelay, Jitter)
- DefaultRetryConfig() with 3 retries, 1s base, 30s max, jitter enabled
- Option type for functional options pattern
- WithHTTPClient, WithTimeout, WithRetryConfig, WithLogger, WithBaseURL

Closes checkvist-api-ymg

client.go

@@ -10,4 +10,74 @@
 //	checklists, err := client.Checklists().List(ctx)
 package checkvist
 
+import (
+	"log/slog"
+	"net/http"
+	"sync"
+	"time"
+)
+
 // client.go contains the Client struct, constructor, and authentication logic.
+
+const (
+	// DefaultBaseURL is the default base URL for the Checkvist API.
+	DefaultBaseURL = "https://checkvist.com"
+	// DefaultTimeout is the default timeout for HTTP requests.
+	DefaultTimeout = 30 * time.Second
+)
+
+// Client is the Checkvist API client.
+type Client struct {
+	// baseURL is the base URL for API requests.
+	baseURL string
+	// username is the user's email address.
+	username string
+	// remoteKey is the API key (remote key) for authentication.
+	remoteKey string
+	// token is the current authentication token.
+	token string
+	// tokenExp is the expiration time of the current token.
+	tokenExp time.Time
+	// httpClient is the HTTP client used for requests.
+	httpClient *http.Client
+	// retryConf is the retry configuration for failed requests.
+	retryConf RetryConfig
+	// logger is the logger for debug and error messages.
+	logger *slog.Logger
+	// mu protects token and tokenExp for concurrent access.
+	mu sync.RWMutex
+}
+
+// NewClient creates a new Checkvist API client.
+//
+// The username should be the user's email address, and remoteKey is the API key
+// which can be obtained from Checkvist settings.
+//
+// Example:
+//
+//	client := checkvist.NewClient("user@example.com", "your-api-key")
+//
+// With options:
+//
+//	client := checkvist.NewClient("user@example.com", "your-api-key",
+//	    checkvist.WithTimeout(60 * time.Second),
+//	    checkvist.WithRetryConfig(checkvist.RetryConfig{MaxRetries: 5}),
+//	)
+func NewClient(username, remoteKey string, opts ...Option) *Client {
+	c := &Client{
+		baseURL:   DefaultBaseURL,
+		username:  username,
+		remoteKey: remoteKey,
+		httpClient: &http.Client{
+			Timeout: DefaultTimeout,
+		},
+		retryConf: DefaultRetryConfig(),
+		logger:    slog.Default(),
+	}
+
+	for _, opt := range opts {
+		opt(c)
+	}
+
+	return c
+}