fix: improve markdown rendering in the chat panel

[Dong1017]

Summary

• improve markdown rendering in the chat panel so narrow layouts remain more readable and closer to Claude Code terminal output
• wrap paragraphs, bullet/task/ordered lists, block quotes, and horizontal rules based on the available message width while preserving prefixes and continuation alignment
• wrap long fenced code block lines to the code block width instead of truncating them
• keep the existing box-table rendering behavior and do not add syntax highlighting in this change

Validation

• ran go test ./ui/panels/...
• verified long paragraphs wrap correctly in a narrow chat panel
• verified bullet, task, and ordered lists preserve prefixes and continuation indentation after wrapping
• verified block quotes remain aligned and fenced code blocks wrap long lines instead of truncating them

Example