Re-home group placement as a category-based auto-assign on the slot booking page

Problem/Motivation

Issue #3612887 removed the manual "place a party together" finder from the placement page in favor of the seat map plus basket and checkout buttons. The group-placement capability (auto-assigning a contiguous set of places for a party) should be re-homed as a booker-facing, category-based auto-assign, not dropped.

Proposed resolution

Surface PlaceAssignment::proposeForCategory() (already used automatically by the pricing provider) as a booker action, most naturally on the slot booking page (/booking/slot/{slot}), where the booker already picks a quantity and a price category. Choosing a quantity for a placed, category-priced event should auto-assign a contiguous run of places in that category, so a party is seated together without picking each seat on the map.

Remaining tasks

• Wire proposeForCategory into the slot booking flow for placed, category-priced resources.
• Warn when the party cannot be seated together (the non-contiguous and short cases proposeForCategory already flags).
• Tests, docs and French translation.