feat: add timezone support with automatic timestamp conversion

Add GetTimezone() API method and WithTimezone() client option.
When enabled, the client lazily fetches the server timezone on
first API call and converts all Timestamp fields in responses
using reflection-based struct walking.

client.go

@@ -4,6 +4,7 @@ import (
 	"log/slog"
 	"net/http"
 	"strings"
+	"sync"
 	"time"
 )
 
@@ -19,6 +20,9 @@ type Client struct {
 	auth           Authenticator
 	logger         *slog.Logger
 	authHeaderName string // custom auth header, empty = use "Authorization"
+	timezone       *time.Location
+	tzOnce         sync.Once
+	tzEnabled      bool
 }
 
 // NewClient creates a new Kanboard API client.
@@ -103,3 +107,11 @@ func (c *Client) WithLogger(logger *slog.Logger) *Client {
 	c.logger = logger
 	return c
 }
+
+// WithTimezone enables automatic timestamp conversion. On the first API call,
+// the client fetches the server's timezone via getTimezone and converts all
+// Timestamp fields in responses to that timezone.
+func (c *Client) WithTimezone() *Client {
+	c.tzEnabled = true
+	return c
+}