Example layout canvas is drag-only — keyboard and touch users cannot fill slots or pass the gated continue

[seekmistar01]

Problem

The example layout canvas on the preview shell (preview/index.html) — the first thing a creator sees in the main browser preview — can only be filled with a mouse via drag-and-drop. The track chips are focusable (tabindex="0"), but there is no keyboard handler, and the drop zones are not keyboard-activatable.

Because the canvas now gates the "Continue to speaker roles" handoff until the host and guest slots are filled, a keyboard-only or touch user can focus a track but has no way to place it — so they cannot pass the gated continue at all. This is an accessibility dead end on the main preview surface.

Why it matters

This is the landing canvas of the browser preview and it gates progression into the episode flow. Drag-and-drop is not operable by keyboard, and HTML5 drag-and-drop is also unreliable on touch. The primary preview should be completable without a mouse.

Proposed fix

Add a select-then-place model alongside drag-and-drop, without removing it:

• Activating a track chip (click or Enter/Space) selects it (with an aria-pressed state) and the status line tells the creator which slot to choose.
• Activating a slot (click or Enter/Space) places the selected track if it matches, mirroring the existing drop validation; activating a slot with nothing selected explains to pick a track first.
• Reset also clears the current selection.

Drag-and-drop, the slot fill status, and the gated continue behaviour stay exactly as they are.