Add board, calendar, gallery, and timeline layouts

## Goal

The higher-affordance layouts Collections need. Board makes status and select workflows obvious. Calendar and timeline anchor rows to dates. Gallery makes image-heavy collections shine. And group-by stops being board-only.

## Scope

* Board layout, grouped by a select, multiselect, or status-like field, with cards showing the preview fields you've picked.
* Calendar layout (month, week, day) anchored on a date field, with drag to reschedule.
* Timeline layout based on a date or date-range field.
* Gallery layout with configurable preview fields and card sizing. Card preview image comes from the row's WP featured image.
* Group-by works on table, list, and gallery too, not just board.

## Out of scope

* Subgrouping (a second-layer group within a layout).
* Calendar event invites or external calendar sync.

## Acceptance criteria

* Each new layout (board, calendar, gallery, timeline) shows up in the view's layout picker and renders rows from the active collection.
* Board groups rows by the field you pick and shows the preview fields on each card.
* Calendar and timeline put rows on the right dates and let you drag to reschedule.
* Gallery cards show the row's featured image and preview fields.
* Group-by works on table, list, and gallery, not just board.

Start at the view's layout picker and the active collection rendering mentioned in the acceptance criteria. Trace how rows expose date fields, select fields, preview fields, and the WP featured image, then verify each layout, drag rescheduling, and group-by behavior against the listed acceptance criteria.

Written by the indexing model from the issue text.