Enhanced log/error visibility in TUI #59

Problem: TUI architecture separates streaming messages from logs; interleaving would require major refactor. Currently, error logs are pinned above streaming messages, feel intrusive, persist until chat reset, and can stack.

Proposed solution: Replace pinned logs with a single, unobtrusive notification (e.g., brief red message) indicating logs are available; allow users to expand on demand via a slash command or shortcut (e.g., Ctrl+L).