feat: add GetColorList for available task colors

Adds GetColorList() to query the available task colors from the
Kanboard instance. Returns a map of color_id to display name
(e.g., "yellow" -> "Yellow").

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

application.go

@@ -0,0 +1,17 @@
+package kanboard
+
+import (
+	"context"
+	"fmt"
+)
+
+// GetColorList returns the available task colors.
+// Returns a map of color_id to display name (e.g., "yellow" -> "Yellow").
+func (c *Client) GetColorList(ctx context.Context) (map[string]string, error) {
+	var result map[string]string
+	if err := c.call(ctx, "getColorList", nil, &result); err != nil {
+		return nil, fmt.Errorf("getColorList: %w", err)
+	}
+
+	return result, nil
+}