feat: add an optional chat bubble

[CatStack-pixe]

Upstream source

• BongoCat PR #985 by @xuebkgithub

Port assessment

The upstream change is a large feature that adds chat UI, local state, HTTP control, and multi-screen behavior. Evaluate and port it as scoped MochiPaw slices; do not expose an unauthenticated local control surface.

Acceptance criteria

• Define the supported chat provider and privacy behavior before implementation.
• Keep the feature optional and disabled by default.
• Review capability, network, and local-server changes before merge.