Complete Client struct with fluent configuration

Add full client configuration with fluent builder pattern:

- WithTimeout(duration) - configurable request timeout
- WithLogger(slog.Logger) - optional debug logging
- DefaultTimeout constant (30 seconds)
- Default HTTP client with timeout on construction

All fluent methods return same client instance for chaining.
Client is thread-safe for concurrent use.

Closes: kanboard-api-uls

client.go

@@ -1,16 +1,23 @@
 package kanboard
 
 import (
+	"log/slog"
 	"net/http"
 	"strings"
+	"time"
 )
 
+// DefaultTimeout is the default HTTP timeout for API requests.
+const DefaultTimeout = 30 * time.Second
+
 // Client is the Kanboard API client.
+// It is safe for concurrent use by multiple goroutines.
 type Client struct {
 	baseURL    string
 	endpoint   string
 	httpClient *http.Client
 	auth       Authenticator
+	logger     *slog.Logger
 }
 
 // NewClient creates a new Kanboard API client.
@@ -22,9 +29,11 @@ func NewClient(baseURL string) *Client {
 	baseURL = strings.TrimSuffix(baseURL, "/")
 
 	return &Client{
-		baseURL:    baseURL,
-		endpoint:   baseURL + "/jsonrpc.php",
-		httpClient: http.DefaultClient,
+		baseURL:  baseURL,
+		endpoint: baseURL + "/jsonrpc.php",
+		httpClient: &http.Client{
+			Timeout: DefaultTimeout,
+		},
 	}
 }
 
@@ -41,7 +50,25 @@ func (c *Client) WithBasicAuth(username, password string) *Client {
 }
 
 // WithHTTPClient sets a custom HTTP client.
+// This replaces the default client entirely, including any timeout settings.
 func (c *Client) WithHTTPClient(client *http.Client) *Client {
 	c.httpClient = client
 	return c
 }
+
+// WithTimeout sets the HTTP client timeout.
+// This creates a new HTTP client with the specified timeout.
+func (c *Client) WithTimeout(timeout time.Duration) *Client {
+	c.httpClient = &http.Client{
+		Timeout:   timeout,
+		Transport: c.httpClient.Transport,
+	}
+	return c
+}
+
+// WithLogger sets the logger for debug output.
+// If set, the client will log request/response details at debug level.
+func (c *Client) WithLogger(logger *slog.Logger) *Client {
+	c.logger = logger
+	return c
+}