feat(tui): add basic UI layout with tab bar and status bar

src/tui/mod.rs

···
 mod app;
 mod ui;
+pub mod widgets;
 
 pub use app::{App, ActiveTab};
+pub use ui::draw;
 
 use std::io;
 use std::panic;

src/tui/ui.rs

···
-use ratatui::Frame;
-use crate::tui::App;
+use ratatui::{
+    layout::{Constraint, Direction, Layout},
+    style::{Color, Style},
+    text::{Line, Span},
+    widgets::{Block, Borders, Paragraph},
+    Frame,
+};
 
-pub fn draw(_frame: &mut Frame, _app: &App) {
-    // Placeholder - will be implemented in Task 4
+use crate::tui::{App, ActiveTab};
+use crate::tui::widgets::TabBar;
+
+pub fn draw(frame: &mut Frame, app: &App) {
+    let chunks = Layout::default()
+        .direction(Direction::Vertical)
+        .constraints([
+            Constraint::Length(1),  // Tab bar
+            Constraint::Min(0),     // Main content
+            Constraint::Length(1),  // Status bar
+        ])
+        .split(frame.area());
+
+    // Tab bar
+    frame.render_widget(TabBar::new(app.active_tab), chunks[0]);
+
+    // Main content area
+    let content_block = Block::default()
+        .borders(Borders::ALL)
+        .title(match app.active_tab {
+            ActiveTab::Dashboard => " Dashboard ",
+            ActiveTab::Planning => " Planning ",
+            ActiveTab::Execution => " Execution ",
+        });
+
+    let placeholder = Paragraph::new(match app.active_tab {
+        ActiveTab::Dashboard => "Dashboard view - coming soon",
+        ActiveTab::Planning => "Planning view - coming soon",
+        ActiveTab::Execution => "Execution view - coming soon",
+    })
+    .block(content_block);
+
+    frame.render_widget(placeholder, chunks[1]);
+
+    // Status bar
+    let status = Line::from(vec![
+        Span::raw(" q quit │ 1/2/3 switch tabs │ Tab cycle │ ? help "),
+    ]);
+    let status_bar = Paragraph::new(status)
+        .style(Style::default().bg(Color::DarkGray));
+    frame.render_widget(status_bar, chunks[2]);
 }