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/service/oauth/manager.rs

@@ -93,7 +93,7 @@ impl OAuthManager {
         let mut url = match url::Url::parse(&self.config.auth_url) {
             Ok(u) if !self.config.auth_url.is_empty() => u,
             _ => {
-                eprintln!(
+                tracing::warn!(
                     "warning: OAuth auth_url is missing or invalid ('{}'); aborting build_auth_url",
                     self.config.auth_url
                 );

src/service/oauth/pkce.rs

@@ -28,7 +28,7 @@ fn rand_byte() -> u8 {
     let nanos = SystemTime::now()
         .duration_since(UNIX_EPOCH)
         .unwrap_or_else(|_| {
-            eprintln!("[pkce] system time before UNIX_EPOCH, using 0 for random byte");
+            tracing::warn!("[pkce] system time before UNIX_EPOCH, using 0 for random byte");
             std::time::Duration::default()
         })
         .subsec_nanos();