Improve backstage queue controls and kiosk QR layout

[mickmister]

Summary

• Removes confirmation prompts when taking someone out of the queue.
• Enables backstage staff to reorder the queue with the existing drag-and-drop queue manager and move controls.
• Adds backstage/internal notes to the walk-in participant form and persists them on manually added participants.
• Nudges the kiosk signup QR layout left so the QR content appears more horizontally centered.

Why

This addresses the people queue usability issues from the task context: removing a performer from the queue should be a quick backstage action, queue order needs to be adjustable during the event, walk-in/ad-hoc entries need the same backstage notes support as edited participants, and the kiosk QR page needed visual centering polish.

Implementation details

• The queued participants section on BackstagePage now renders QueueManager, which already supports drag-and-drop reordering via @dnd-kit and up/down controls backed by reorderQueue.
• Drag listeners were moved to the grip handle so normal row interactions do not accidentally initiate a drag.
• removeFromQueue actions are now called directly from both queue UIs without a browser confirm dialog.
• addManualParticipant accepts an optional notes field, and the walk-in form tracks, saves, and clears backstage notes.
• The kiosk QR container uses border-box sizing plus a small responsive left transform to better center the QR group visually.

Validation

• CI=true pnpm build