[Desktop] Reactions sometimes disappear from the UI

> [!IMPORTANT]
> Do not include security vulnerabilities in a public issue. [Report them
> privately through a GitHub security advisory](https://github.com/block/buzz/security/advisories/new).

**Describe the bug**
Reactions sent to messages sometimes simply disappear from the UI.

**Steps to reproduce**
Not sure how. Add a few reactions to any message, and restart Buzz Desktop a few times.

When a previously sent reaction disappears from the UI, re-adding the reaction gives:
"relay rejected event: duplicate: reaction already exists"
This should indicate the bug is on the UI, not anything related to the relay.

**Expected behavior**
Reacts should not randomly disappear.

**Version and platform**
Find your version at the bottom of the Settings sidebar. Write "unknown" if you can't determine it.
- Buzz version: v0.5.23
- OS: ArchLinux (Linux 7.0 + NVIDIA)

**Logs / additional context**

This is what happens when re-adding a thumbs-up react to some message in a channel:

Start by reproducing the issue in Buzz Desktop v0.5.23 on ArchLinux: add reactions to a message, restart the app several times, and compare the UI with the duplicate-event response when re-adding a missing reaction. Trace the desktop reaction display and reload path; done means previously sent reactions remain visible after restarts and are not incorrectly offered for re-adding.

Written by the indexing model from the issue text.