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/main.rs

@@ -1,5 +1,6 @@
 use std::io;
 use std::io::Write;
+use std::sync::Mutex;
 use anyhow::Result;
 use crossterm::execute;
 use crossterm::terminal::{disable_raw_mode, enable_raw_mode, EnterAlternateScreen, LeaveAlternateScreen};
@@ -23,11 +24,26 @@ fn main() -> Result<()> {
         .position(|a| a == "--attach")
         .and_then(|i| args.get(i + 1).cloned());
 
+    let log_dir = dirs::data_dir()
+        .unwrap_or_else(|| std::path::PathBuf::from("."))
+        .join("zesdex");
+    let _ = std::fs::create_dir_all(&log_dir);
+    let log_path = log_dir.join("zesdex.log");
+    let log_file = std::fs::OpenOptions::new()
+        .create(true).append(true).open(&log_path)
+        .unwrap_or_else(|_| {
+            // Fallback: /dev/null so the TUI isn't corrupted by stderr writes
+            std::fs::OpenOptions::new()
+                .write(true).open("/dev/null")
+                .expect("cannot open /dev/null")
+        });
+
     tracing_subscriber::fmt()
         .with_env_filter(
             tracing_subscriber::EnvFilter::try_from_default_env()
                 .unwrap_or_else(|_| tracing_subscriber::EnvFilter::new("info")),
         )
+        .with_writer(Mutex::new(log_file))
         .init();
 
     if is_daemon && attach_session.is_some() {