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/view/mod.rs

@@ -36,6 +36,9 @@ pub fn draw(frame: &mut Frame, state: &crate::app::state::rest::AppStateRest) {
 
     render_input_bar(frame, input_area, state);
     status::draw_status_bar(frame, status_area, state);
+
+    // Toast notifications at top-right (like Hyprland)
+    render_toasts(frame, state);
 }
 
 fn render_main_panel(frame: &mut Frame, area: Rect, state: &crate::app::state::rest::AppStateRest) {
@@ -564,6 +567,49 @@ fn render_input_bar(frame: &mut Frame, area: Rect, state: &crate::app::state::re
     frame.render_widget(paragraph, area);
 }
 
+/// Render active toasts as a floating stack at top-right of the terminal.
+/// Each toast auto-expires after its lifetime_ms. Max 4 visible at once.
+fn render_toasts(frame: &mut Frame, state: &crate::app::state::rest::AppStateRest) {
+    let now_ms = chrono::Utc::now().timestamp_millis();
+    let active: Vec<&crate::app::state::types::Toast> = state.misc.toasts.iter()
+        .filter(|t| !t.expired(now_ms))
+        .collect();
+    if active.is_empty() {
+        return;
+    }
+    let area = frame.area();
+    let toast_w: u16 = 45;
+    let x = area.width.saturating_sub(toast_w).saturating_sub(2);
+    let mut y: u16 = 1;
+
+    for toast in active.iter().rev().take(4) {
+        let line_count = toast.message.lines().count().max(1) as u16;
+        let h = line_count + 2; // border top + border bottom
+        let toast_area = Rect { x, y, width: toast_w, height: h };
+        if toast_area.bottom() > area.height {
+            break;
+        }
+        frame.render_widget(ratatui::widgets::Clear, toast_area);
+
+        let border_color = match toast.kind {
+            crate::app::state::types::ToastKind::Success => Theme::SUCCESS,
+            crate::app::state::types::ToastKind::Warning => Theme::WARNING,
+            crate::app::state::types::ToastKind::Error => Theme::ERROR,
+            crate::app::state::types::ToastKind::Info => Theme::INFO,
+            crate::app::state::types::ToastKind::Lesson => Theme::PRIMARY,
+        };
+        let block = ratatui::widgets::Block::default()
+            .borders(ratatui::widgets::Borders::ALL)
+            .border_style(ratatui::style::Style::default().fg(border_color))
+            .style(ratatui::style::Style::default().bg(ratatui::style::Color::Black));
+        let paragraph = ratatui::widgets::Paragraph::new(toast.message.as_str())
+            .block(block)
+            .wrap(ratatui::widgets::Wrap { trim: false });
+        frame.render_widget(paragraph, toast_area);
+        y = y.saturating_add(h).saturating_add(1);
+    }
+}
+
 fn centered_rect(area: Rect, percent_x: u16, percent_y: u16) -> Rect {
     let x_pad = (area.width.saturating_sub(area.width * percent_x / 100)) / 2;
     let y_pad = (area.height.saturating_sub(area.height * percent_y / 100)) / 2;