Channel Background Images (Like IOS Messages)

**Motivation**
Gives the channels a little more personality, distinguishability.

**Proposed solution**
I'd like to set a background for the channel chat or have an agent set it for me by mentioning it.

**Alternatives considered**
None

**Additional context**
None

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Before opening: please [search open issues and PRs](https://github.com/block/buzz/issues?q=is%3Aopen) for duplicates — link the closest one, or say "none found".

The issue identifies no files, tests, or entry points, so first locate the channel chat UI and any existing channel or agent settings flow. Clarify whether backgrounds are user-configurable, agent-controlled, or both, then define the supported behavior and acceptance criteria before implementation.

Written by the indexing model from the issue text.