feat(desktop): collapse very long messages behind a "Show more" toggle

Long messages render at full height in the channel, so a single message can push everything else off screen and make scrolling back through a conversation painful. Agent output in particular is often hundreds of lines.

**Proposal:** clamp a message to a fixed max height (Slack uses ~a dozen lines) and show a "Show more" / "Show less" toggle when it overflows. Expansion state is per-message and not persisted.

**Notes**
- Applies to both human and agent messages; same treatment for thread previews.
- Fenced code blocks should be included in the clamp, not exempt from it.
- Search/keyboard navigation should auto-expand a match so it is not hidden behind the fold.

Start by locating the desktop message renderer and the code handling thread previews, then trace search and keyboard-navigation matches. Implement the per-message collapse state for human and agent messages, including fenced code blocks, with Show more/Show less controls and automatic expansion for navigated matches. Done means long messages no longer dominate the channel while expanded messages remain readable.

Written by the indexing model from the issue text.