refactor: migrate to DDD layered architecture

Split the monolithic app.rs (762 lines) into four DDD layers:
- domain/: models, navigation enums (Tab, ModalKind), conversions, aggregates
- application/: App state, LibraryState, NotificationManager, event handlers
- infrastructure/: config, gRPC client, system utilities
- presentation/: all render functions, widgets, views, modals

Original modules (app, data, ui, config, grpc) preserved as thin
re-export facades for backward compatibility. All 13 insta snapshot
tests pass without modification.

src/presentation/notifications.rs

@@ -0,0 +1,146 @@
+#![allow(dead_code)]
+
+use std::time::Instant;
+
+use ratatui::{
+    Frame,
+    layout::Rect,
+    style::Style,
+    text::{Line, Span},
+    widgets::{Block, Borders, Clear, Paragraph},
+};
+
+use crate::application::notification_state::{format_elapsed, Notification, NotificationManager, MAX_VISIBLE};
+use crate::theme;
+
+impl NotificationManager {
+    pub fn render(&self, frame: &mut Frame, area: Rect) {
+        let visible: Vec<&Notification> = self
+            .active()
+            .iter()
+            .rev()
+            .take(MAX_VISIBLE)
+            .collect::<Vec<_>>()
+            .into_iter()
+            .rev()
+            .collect();
+
+        if visible.is_empty() {
+            return;
+        }
+
+        let notif_width = 50u16.min(area.width.saturating_sub(4));
+        let notif_height = 3u16;
+        let spacing = 1u16;
+        let total_height = visible.len() as u16 * (notif_height + spacing);
+
+        let start_y = area.y + area.height.saturating_sub(total_height + 1);
+        let start_x = area.x + area.width.saturating_sub(notif_width + 2);
+
+        for (i, notif) in visible.iter().enumerate() {
+            let y = start_y + (i as u16) * (notif_height + spacing);
+            let notif_area = Rect::new(start_x, y, notif_width, notif_height);
+
+            frame.render_widget(Clear, notif_area);
+
+            let border_color = notif.kind.color();
+            let block = Block::default()
+                .borders(Borders::LEFT)
+                .border_style(Style::default().fg(border_color))
+                .style(Style::default().bg(theme::BG1));
+
+            let inner = block.inner(notif_area);
+            frame.render_widget(block, notif_area);
+
+            let elapsed = Instant::now().duration_since(notif.created_at).as_secs();
+            let timestamp = if elapsed == 0 {
+                "now".to_string()
+            } else {
+                format!("{}s", elapsed)
+            };
+
+            let mut lines = vec![Line::from(vec![
+                Span::styled(&notif.icon, Style::default().fg(border_color)),
+                Span::raw(" "),
+                Span::styled(
+                    &notif.title,
+                    Style::default()
+                        .fg(theme::FG1)
+                        .add_modifier(ratatui::style::Modifier::BOLD),
+                ),
+                Span::raw(" "),
+                Span::styled(timestamp, Style::default().fg(theme::GRAY)),
+            ])];
+
+            if let Some(detail) = &notif.detail {
+                let max_len = inner.width.saturating_sub(2) as usize;
+                let d = if detail.len() > max_len {
+                    format!("{}…", &detail[..max_len.saturating_sub(1)])
+                } else {
+                    detail.clone()
+                };
+                lines.push(Line::from(Span::styled(
+                    d,
+                    Style::default().fg(theme::GRAY),
+                )));
+            }
+
+            let para = Paragraph::new(lines);
+            frame.render_widget(para, inner);
+        }
+    }
+}
+
+pub fn render_notification_item(frame: &mut Frame, area: Rect, notif: &Notification) {
+    let border_color = notif.kind.color();
+    let block = Block::default()
+        .borders(Borders::LEFT)
+        .border_style(Style::default().fg(border_color))
+        .style(Style::default().bg(theme::BG1));
+
+    let inner = block.inner(area);
+    frame.render_widget(block, area);
+
+    let elapsed = Instant::now().duration_since(notif.created_at).as_secs();
+    let timestamp = format_elapsed(elapsed);
+
+    let mut lines = vec![Line::from(vec![
+        Span::styled(&notif.icon, Style::default().fg(border_color)),
+        Span::raw(" "),
+        Span::styled(
+            notif.kind.label(),
+            Style::default()
+                .fg(theme::FG1)
+                .add_modifier(ratatui::style::Modifier::BOLD),
+        ),
+        Span::raw(" "),
+        Span::styled(timestamp, Style::default().fg(theme::GRAY)),
+    ])];
+
+    if let Some(detail) = &notif.detail {
+        let max_len = inner.width.saturating_sub(2) as usize;
+        let d = if detail.len() > max_len {
+            format!("{}…", &detail[..max_len.saturating_sub(1)])
+        } else {
+            detail.clone()
+        };
+        lines.push(Line::from(Span::styled(
+            d,
+            Style::default().fg(theme::GRAY),
+        )));
+    } else {
+        let max_len = inner.width.saturating_sub(2) as usize;
+        let title = if notif.title.len() > max_len {
+            format!("{}…", &notif.title[..max_len.saturating_sub(1)])
+        } else {
+            notif.title.clone()
+        };
+        lines.push(Line::from(Span::styled(
+            title,
+            Style::default().fg(theme::GRAY),
+        )));
+    }
+
+    let para = Paragraph::new(lines);
+    frame.render_widget(para, inner);
+}