[Bug] Scope message selection to the active conversation

In the macOS desktop app, native text selection can include content outside the user's intended scope:

- In split view, dragging from channel content into a thread selects both channel and thread content.
- Dragging across a message can select hover quick-reaction emojis and inline reaction pills.
- Popup/overlay content can also participate in selection.

**Steps to reproduce**

1. Open a channel with a threaded message.
2. Open the thread in split view.
3. Drag-select from a channel message into a thread reply.
4. Hover a message and drag-select across its action area.
5. Repeat on a message with reaction pills.

**Expected behavior**

- Channel selection remains inside the channel.
- Thread selection remains inside the thread.
- Message text remains copyable.
- Hover action bars, quick reactions, reaction pills, emoji pickers, reaction popovers, and other interactive overlays are excluded.
- Timestamps and ordinary message metadata may remain selectable.

**Version and platform**

- Buzz desktop source/config version: `0.5.3`
- Source commit: `b1b283cd4`
- macOS `26.6` (build `25G5065a`), Apple Silicon `arm64`

**Logs / additional context**

Current selection behavior example: