fix(composer): serialize sends and snapshot drafts across the composer

[eleboucher]

Description

sends weren't serialized so a slow one could interleave with the next, and the editor cleared before the send resolved so a failure lost your draft.

sends go through a per-composer queue now, draft and reply are snapshotted on submit and put back if it fails.

attachments, polls, location and audio all send through one onSubmit so they share the queue. location and poll dialogs show errors and disable while sending.

sendDelayedMessage takes an event type instead of hardcoding m.room.message.

Fixes #

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).

[github-actions]

Deploying with    Cloudflare Workers

Status	Preview URL	Commit	Alias	Updated (UTC)
✅ Deployment successful!	https://pr-1567-sable.raspy-dream-bb1d.workers.dev	31e30c7	pr-1567	Thu, 06 Aug 2026 03:13:26 GMT