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/app/subagent/engine.rs

@@ -32,7 +32,7 @@ fn resolve_provider_config() -> (String, String, Option<String>) {
     let app_config = crate::model::app_config::AppConfig::load();
 
     let mut api_key = settings.api_keys.get(&settings.provider).cloned().unwrap_or_else(|| {
-        eprintln!("[subagent] no API key for provider '{}' in settings, trying env/default", settings.provider);
+        tracing::warn!("[subagent] no API key for provider '{}' in settings, trying env/default", settings.provider);
         String::new()
     });
     let model = settings.model.clone();
@@ -45,7 +45,7 @@ fn resolve_provider_config() -> (String, String, Option<String>) {
                 .and_then(|env| std::env::var(env).ok())
                 .or_else(|| provider_cfg.default_api_key.clone())
                 .unwrap_or_else(|| {
-                    eprintln!("[subagent] all API key resolution paths exhausted for '{}'", settings.provider);
+                    tracing::warn!("[subagent] all API key resolution paths exhausted for '{}'", settings.provider);
                     String::new()
                 });
         }