- Replace all eprintln! with tracing::warn! to avoid TUI corruption
via stderr writes during alternate screen mode
- Route tracing output to ~/.local/share/zesdex/zesdex.log instead of
stderr by configuring tracing_subscriber with a Mutex<File> writer
- Add render_toasts() widget: floating notification stack at top-right
of the terminal, color-coded by severity (Info/Success/Warning/Error/
Lesson), auto-expires after 5s, max 4 visible
- Apply to 12 files: stream parser, MCP client, subagent engine,
state/rest, controller/input, app_config, provider, OAuth, agent_def,
dto/chat/tool
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>