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

options.go

@@ -1,3 +1,73 @@
 package checkvist
 
+import (
+	"log/slog"
+	"net/http"
+	"time"
+)
+
 // options.go contains functional options for configuring the Client.
+
+// RetryConfig configures the retry behavior for failed requests.
+type RetryConfig struct {
+	// MaxRetries is the maximum number of retry attempts.
+	MaxRetries int
+	// BaseDelay is the initial delay before the first retry.
+	BaseDelay time.Duration
+	// MaxDelay is the maximum delay between retries.
+	MaxDelay time.Duration
+	// Jitter enables randomized delay to prevent thundering herd.
+	Jitter bool
+}
+
+// DefaultRetryConfig returns the default retry configuration.
+func DefaultRetryConfig() RetryConfig {
+	return RetryConfig{
+		MaxRetries: 3,
+		BaseDelay:  1 * time.Second,
+		MaxDelay:   30 * time.Second,
+		Jitter:     true,
+	}
+}
+
+// Option is a functional option for configuring the Client.
+type Option func(*Client)
+
+// WithHTTPClient sets a custom HTTP client for the Checkvist client.
+func WithHTTPClient(client *http.Client) Option {
+	return func(c *Client) {
+		c.httpClient = client
+	}
+}
+
+// WithTimeout sets the timeout for HTTP requests.
+// This creates a new HTTP client with the specified timeout.
+func WithTimeout(timeout time.Duration) Option {
+	return func(c *Client) {
+		c.httpClient = &http.Client{
+			Timeout: timeout,
+		}
+	}
+}
+
+// WithRetryConfig sets the retry configuration for failed requests.
+func WithRetryConfig(config RetryConfig) Option {
+	return func(c *Client) {
+		c.retryConf = config
+	}
+}
+
+// WithLogger sets a custom logger for the client.
+func WithLogger(logger *slog.Logger) Option {
+	return func(c *Client) {
+		c.logger = logger
+	}
+}
+
+// WithBaseURL sets a custom base URL for the API.
+// This is primarily useful for testing.
+func WithBaseURL(url string) Option {
+	return func(c *Client) {
+		c.baseURL = url
+	}
+}