fix: route warnings to file and add in-app toast notifications

- 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>

src/controller/input.rs

@@ -245,7 +245,7 @@ fn handle_overlay_enter(state: &mut AppStateRest) -> Vec<Action> {
             if let Some(provider) = providers.get(state.misc.selected_index) {
                 if let Some(cfg) = state.app_config.providers.get(provider) {
                     let model = cfg.default_model.clone().unwrap_or_else(|| {
-                        eprintln!("[input] provider '{}' has no default_model, using 'claude-opus-4-8'", provider);
+                        tracing::warn!("[input] provider '{}' has no default_model, using 'claude-opus-4-8'", provider);
                         "claude-opus-4-8".to_string()
                     });
                     state.settings.provider = provider.clone();