dayHeaderBuilder property

Optional builder for fully custom day/column header widgets (#79). When non-null the planner replaces the painted DateRow text with a row of host-built header widgets — one per config.labels entry — each sized to the column's blockWidth and the date row's height, and offset by the live horizontal scroll so the headers track the day-columns below as the user pans (rebuilt on the same triggerUpdate the row repaints on).

The header row is wrapped in IgnorePointer, so a horizontal drag across it still pans the day axis through the same gesture detector as the painted row — the builder decides appearance only. When null (the default) the painted DateRow is shown exactly as before.