diff --git a/crates/nuncio-tui/src/layout.rs b/crates/nuncio-tui/src/layout.rs
new file mode 100644
index 0000000..d4234e7
--- /dev/null
+++ b/crates/nuncio-tui/src/layout.rs
@@ -0,0 +1,99 @@
+//! Multi-pane Ratatui layout split and focus management for terminal UI.
+
+use ratatui::layout::{Constraint, Direction, Layout, Rect};
+use ratatui::style::{Color, Style};
+
+/// Active focused pane in the terminal UI.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+pub enum ActivePane {
+    /// Folder navigation sidebar (left 20%).
+    #[default]
+    Sidebar,
+    /// Message list table (middle 35%).
+    MessageList,
+    /// Envelope preview reader (right 45%).
+    Reader,
+}
+
+impl ActivePane {
+    /// Cycle focus to the next pane (tab/right key).
+    pub fn next(self) -> Self {
+        match self {
+            Self::Sidebar => Self::MessageList,
+            Self::MessageList => Self::Reader,
+            Self::Reader => Self::Sidebar,
+        }
+    }
+
+    /// Cycle focus to the previous pane (shift-tab/left key).
+    pub fn previous(self) -> Self {
+        match self {
+            Self::Sidebar => Self::Reader,
+            Self::MessageList => Self::Sidebar,
+            Self::Reader => Self::MessageList,
+        }
+    }
+}
+
+/// Layout engine computing 3-pane responsive terminal view bounds.
+pub struct AppLayout;
+
+impl AppLayout {
+    /// Compute 3-pane horizontal split: (sidebar, message_list, reader).
+    pub fn compute_layout(area: Rect) -> (Rect, Rect, Rect) {
+        let chunks = Layout::default()
+            .direction(Direction::Horizontal)
+            .constraints([
+                Constraint::Percentage(20),
+                Constraint::Percentage(35),
+                Constraint::Percentage(45),
+            ])
+            .split(area);
+
+        (chunks[0], chunks[1], chunks[2])
+    }
+
+    /// Compute block border style for active vs inactive panes.
+    pub fn border_style(pane: ActivePane, current: ActivePane) -> Style {
+        if pane == current {
+            Style::default().fg(Color::Yellow)
+        } else {
+            Style::default().fg(Color::DarkGray)
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn active_pane_focus_cycling() {
+        let pane = ActivePane::Sidebar;
+        assert_eq!(pane.next(), ActivePane::MessageList);
+        assert_eq!(pane.next().next(), ActivePane::Reader);
+        assert_eq!(pane.next().next().next(), ActivePane::Sidebar);
+
+        assert_eq!(pane.previous(), ActivePane::Reader);
+        assert_eq!(pane.previous().previous(), ActivePane::MessageList);
+    }
+
+    #[test]
+    fn compute_layout_splits_area_three_ways() {
+        let area = Rect::new(0, 0, 100, 40);
+        let (sidebar, list, reader) = AppLayout::compute_layout(area);
+
+        assert_eq!(sidebar.width, 20);
+        assert_eq!(list.width, 35);
+        assert_eq!(reader.width, 45);
+    }
+
+    #[test]
+    fn border_style_highlights_focused_pane() {
+        let focused = AppLayout::border_style(ActivePane::Sidebar, ActivePane::Sidebar);
+        let unfocused = AppLayout::border_style(ActivePane::Sidebar, ActivePane::Reader);
+
+        assert_eq!(focused.fg, Some(Color::Yellow));
+        assert_eq!(unfocused.fg, Some(Color::DarkGray));
+    }
+}
diff --git a/crates/nuncio-tui/src/main.rs b/crates/nuncio-tui/src/main.rs
index cf82452..eb79425 100644
--- a/crates/nuncio-tui/src/main.rs
+++ b/crates/nuncio-tui/src/main.rs
@@ -1,8 +1,17 @@
-use nuncio_core::NuncioEngine;
+//! Nuncio Terminal UI application entry point.
 
-#[tokio::main]
-async fn main() -> Result<(), Box<dyn std::error::Error>> {
-    let engine = NuncioEngine::new();
-    println!("Starting {} TUI shell ({})", engine.name, engine.domain);
-    Ok(())
+mod layout;
+
+use layout::{ActivePane, AppLayout};
+use ratatui::layout::Rect;
+
+fn main() {
+    let area = Rect::new(0, 0, 120, 40);
+    let (sidebar, list, reader) = AppLayout::compute_layout(area);
+    let focused = ActivePane::Sidebar;
+
+    println!(
+        "Nuncio TUI Layout Active: sidebar={}px, list={}px, reader={}px (focused: {:?})",
+        sidebar.width, list.width, reader.width, focused
+    );
 }