fix: render Markdown in conversation viewer

[zeroknots]

Summary

Render assistant messages and tool results in the FleetView conversation overlay with Pi's Markdown component. This fixes raw Markdown output from tools such as context-mode's ctx_execute and removes the fixed 500-character result truncation so the existing scrollable viewer can show the complete result.

Closes #210.

Changes

• Add viewer-local Markdown styling based on the overlay theme.
• Render assistant text and tool results through Pi's Markdown component.
• Preserve width clamping and existing conversation chrome.
• Add regression coverage for headings, lists, inline emphasis, and tool results beyond 500 characters.

Testing

• npm run lint
• npm run typecheck
• npm run test — 47 files passed, 1 skipped; 822 tests passed, 5 skipped
• npm run build