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/model/app_config.rs

@@ -64,7 +64,7 @@ impl AppConfig {
             Ok(s) => match serde_json::from_str(&s) {
                 Ok(c) => c,
                 Err(e) => {
-                    eprintln!(
+                    tracing::warn!(
                         "warning: failed to parse config file '{}': {}. Loading defaults.",
                         path.display(), e
                     );