Placement page: keep only the seat map, add basket and checkout buttons

Problem/Motivation

The public place picker (/placement/{slot}) stacks a graphical seat map plus three form sections: a "Place map" text list, a "Place a party together" adjacent-seats finder, and a "Pick a single place" select. The extra sections clutter the page. The graphical seat map is the intended way to choose seats, and from there a booker should be able to go to the basket or straight to checkout.

Proposed resolution

Keep only the graphical seat map and remove the three form sections. In their place add two buttons: "Go to basket" (links to the cart) and "Checkout" (runs the same checkout as the cart page). The buttons appear only when the cart module is installed, so the placement module keeps no hard dependency on it.

• Simplify PlaceBookingForm to the seat map plus the two actions; remove the now-dead handlers and helpers.
• Drop the unused proposeAdjacent() and candidatePlaces() from PlaceAssignment; keep the per-category auto-assign used by the pricing provider.
• Update tests, docs and the French translation.

Remaining tasks

• Implement, test, review.

Follow-up: re-home a booker-facing group placement as a category-based auto-assign, in a separate issue.