* Keep unfinished canvas edits when a teammate's change streams in

A remote snapshot reloads the whole active page. The safety check waited
for unsaved work and text drafts but not for a stroke, drag, resize, or
lineup placement still in progress, so the reload wiped it.

The check now also waits while a pointer or trackpad gesture is held,
a drawing or lineup placement is unfinished, or a text draft is open,
and applies the latest snapshot once the edit ends. The reload re-checks
after its async load and yields to a page switch that started meanwhile.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* Check the held-back remote change applies once the stroke lands

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>