feat: add persistent server-client sessions

   Make herdr persistent by default.

   Launching herdr now starts or reattaches to a background session
   server. Clients can detach and reattach while panes and agent
   processes keep running. Session mode now supports multi-client attach,
   auto-detect startup, and a thin-client/headless-server split.

   This also refactors the large app, ui, input, pane, config,
   workspace, and persistence modules into smaller focused submodules,
   while preserving behavior and colocating tests with the code they
   exercise.

   Upgrade notes:
   - persistence mode is now the default
   - in-app quit detaches the current client instead of stopping the server
   - use `herdr server stop` to stop the background session
   - use `--no-session` for the old single-process behavior
   - default socket paths now live under the config directory

1 files changed, 238 insertions, 0 deletions

diff --git a/src/ui/settings.rs b/src/ui/settings.rs
new file mode 100644
index 00000000..1123f408
--- /dev/null
+++ b/src/ui/settings.rs
@@ -0,0 +1,238 @@
+use ratatui::{
+    layout::{Constraint, Layout, Rect},
+    style::{Modifier, Style},
+    text::{Line, Span},
+    widgets::{List, ListItem, ListState, Paragraph, Tabs},
+    Frame,
+};
+
+use super::widgets::{
+    action_button_row_rects, centered_popup_rect, modal_stack_areas, panel_contrast_fg,
+    render_action_button, render_panel_shell, ActionButtonSpec,
+};
+use crate::app::{state::Palette, AppState};
+
+pub(super) fn render_settings_overlay(app: &AppState, frame: &mut Frame, area: Rect) {
+    use crate::app::state::SettingsSection;
+
+    let p = &app.palette;
+    let Some(popup) = centered_popup_rect(area, 56, 20) else {
+        return;
+    };
+
+    super::dim_background(frame, area);
+
+    let Some(inner) = render_panel_shell(frame, popup, p.accent, p.panel_bg) else {
+        return;
+    };
+    if inner.height < 4 || inner.width < 10 {
+        return;
+    }
+
+    let stack = modal_stack_areas(inner, 3, 2, 0, 1);
+    let header_rows = Layout::vertical([
+        Constraint::Length(1),
+        Constraint::Length(1),
+        Constraint::Length(1),
+    ])
+    .areas::<3>(stack.header);
+
+    frame.render_widget(
+        Paragraph::new(Line::from(vec![Span::styled(
+            " settings",
+            Style::default().fg(p.text).add_modifier(Modifier::BOLD),
+        )])),
+        header_rows[0],
+    );
+
+    let tabs = Tabs::new(SettingsSection::ALL.iter().map(|s| s.label()))
+        .select(
+            SettingsSection::ALL
+                .iter()
+                .position(|section| *section == app.settings.section)
+                .unwrap_or(0),
+        )
+        .style(Style::default().fg(p.overlay1))
+        .highlight_style(
+            Style::default()
+                .fg(panel_contrast_fg(p))
+                .bg(p.accent)
+                .add_modifier(Modifier::BOLD),
+        )
+        .divider(" ")
+        .padding(" ", " ");
+    frame.render_widget(tabs, header_rows[1]);
+
+    let sep = "─".repeat(inner.width as usize);
+    frame.render_widget(
+        Paragraph::new(Span::styled(&sep, Style::default().fg(p.surface0))),
+        header_rows[2],
+    );
+
+    let content_area = stack.content;
+
+    match app.settings.section {
+        SettingsSection::Theme => {
+            render_settings_theme(app, frame, content_area);
+        }
+        SettingsSection::Sound => {
+            render_settings_toggle(
+                frame,
+                content_area,
+                p,
+                "sound alerts",
+                "play sounds when agents change state in background",
+                app.sound_enabled(),
+                app.settings.list.selected,
+            );
+        }
+        SettingsSection::Toast => {
+            render_settings_toggle(
+                frame,
+                content_area,
+                p,
+                "visual toasts",
+                "show top-right notifications for background events",
+                app.toast_config.enabled,
+                app.settings.list.selected,
+            );
+        }
+    }
+
+    if let Some(footer_area) = stack.footer {
+        let footer_rows = Layout::vertical([Constraint::Length(1), Constraint::Length(1)])
+            .areas::<2>(footer_area);
+        let (apply_rect, close_rect) = settings_button_rects(inner);
+        render_action_button(
+            frame,
+            apply_rect,
+            Some("↵"),
+            "apply",
+            Style::default()
+                .fg(panel_contrast_fg(p))
+                .bg(p.accent)
+                .add_modifier(Modifier::BOLD),
+        );
+        render_action_button(
+            frame,
+            close_rect,
+            Some("esc"),
+            "close",
+            Style::default()
+                .fg(p.text)
+                .bg(p.surface0)
+                .add_modifier(Modifier::BOLD),
+        );
+
+        frame.render_widget(
+            Paragraph::new(Line::from(vec![
+                Span::styled(" ↑↓", Style::default().fg(p.overlay0)),
+                Span::styled(" select  ", Style::default().fg(p.overlay1)),
+                Span::styled("tab", Style::default().fg(p.overlay0)),
+                Span::styled(" section", Style::default().fg(p.overlay1)),
+            ])),
+            footer_rows[0],
+        );
+    }
+}
+
+pub(crate) fn settings_button_rects(inner: Rect) -> (Rect, Rect) {
+    let rects = action_button_row_rects(
+        inner,
+        &[
+            ActionButtonSpec {
+                hint: Some("↵"),
+                label: "apply",
+            },
+            ActionButtonSpec {
+                hint: Some("esc"),
+                label: "close",
+            },
+        ],
+        2,
+        inner.height.saturating_sub(1),
+    );
+    (rects[0], rects[1])
+}
+
+fn render_settings_theme(app: &AppState, frame: &mut Frame, area: Rect) {
+    use crate::app::state::THEME_NAMES;
+
+    let p = &app.palette;
+    let items: Vec<ListItem> = THEME_NAMES
+        .iter()
+        .map(|name| {
+            let is_current = name.to_lowercase().replace([' ', '_'], "-")
+                == app.theme_name.to_lowercase().replace([' ', '_'], "-");
+            let marker = if is_current { " ✓" } else { "" };
+            ListItem::new(Line::from(vec![
+                Span::styled(*name, Style::default().fg(p.subtext0)),
+                Span::styled(marker, Style::default().fg(p.green)),
+            ]))
+        })
+        .collect();
+
+    let list = List::new(items)
+        .highlight_style(
+            Style::default()
+                .bg(p.surface0)
+                .fg(p.text)
+                .add_modifier(Modifier::BOLD),
+        )
+        .highlight_symbol(" ▸ ")
+        .style(Style::default().fg(p.subtext0));
+
+    let mut state = ListState::default().with_selected(Some(app.settings.list.selected));
+    frame.render_stateful_widget(list, area, &mut state);
+}
+
+fn render_settings_toggle(
+    frame: &mut Frame,
+    area: Rect,
+    p: &Palette,
+    title: &str,
+    description: &str,
+    current_value: bool,
+    selected_idx: usize,
+) {
+    let [desc_area, _, list_area] = Layout::vertical([
+        Constraint::Length(1),
+        Constraint::Length(1),
+        Constraint::Min(2),
+    ])
+    .areas::<3>(area);
+
+    let max_desc_len = (desc_area.width as usize).saturating_sub(2);
+    let desc_text = if description.len() > max_desc_len {
+        format!(" {}…", &description[..max_desc_len.saturating_sub(2)])
+    } else {
+        format!(" {description}")
+    };
+    frame.render_widget(
+        Paragraph::new(Span::styled(desc_text, Style::default().fg(p.overlay1))),
+        desc_area,
+    );
+
+    let items: Vec<ListItem> = ["on", "off"]
+        .into_iter()
+        .map(|label| {
+            let is_active = (label == "on") == current_value;
+            let marker = if is_active { " ✓" } else { "" };
+            ListItem::new(Line::from(vec![
+                Span::styled(format!("{title}: {label}"), Style::default().fg(p.subtext0)),
+                Span::styled(marker, Style::default().fg(p.green)),
+            ]))
+        })
+        .collect();
+
+    let list = List::new(items)
+        .highlight_style(
+            Style::default()
+                .bg(p.surface0)
+                .fg(p.text)
+                .add_modifier(Modifier::BOLD),
+        )
+        .highlight_symbol(" ▸ ");
+    let mut state = ListState::default().with_selected(Some(selected_idx.min(1)));
+    frame.render_stateful_widget(list, list_area, &mut state);
+}