No agent typing or activity indicator in forum threads

**Describe the bug**

When an agent is working on a mention inside a forum thread, the thread shows no activity at all. No typing indicator, no thinking state, nothing to clear when the reply lands. It just sits there until the answer shows up, so on a long turn you cannot tell whether the agent picked the message up or ignored it. Stream channels show all of this.

The publishing side works correctly. `crates/buzz-acp/src/relay.rs` emits `KIND_TYPING_INDICATOR` (20002) and scopes it to the thread with a reply tag:

```rust
tags.push(Tag::parse(["e", parent, "", "reply"])?);
let event = EventBuilder::new(Kind::Custom(KIND_TYPING_INDICATOR as u16), "")
```

The consuming side exists only for streams: `desktop/src/features/messages/useChannelTyping.ts`. The forum feature has no typing handling anywhere. `ForumView.tsx`, `ForumThreadPanel.tsx` and `ForumComposer.tsx` contain no reference to typing, presence, or activity state. The events are published with correct thread scoping and then dropped.

The same holds for agent observer frames (`KIND_AGENT_OBSERVER_FRAME`), which are consumed only in the local-archive and observer-relay paths, never in the forum view.

**Steps to reproduce**

1. Add a managed agent to a forum channel
2. Mention it inside a thread so it starts a turn
3. Watch the thread while it works

**Expected behavior**

The same activity feedback the stream view gives: an indication that the agent has picked the message up, a working state while the turn runs, and clearing when the reply is posted. The events already carry the thread id, so the data needed to render this per thread is present.

**Version and platform**

- Buzz version: 0.5.6
- OS: Windows 11 (10.0.26200)

**Additional context**

Only observable after working around the forum mention routing issue filed separately, since without that agents never receive forum mentions at all.