feat(tui): single-dashboard layout with sidebar and overlay

Replace 3-tab navigation with a single monitoring dashboard.

- Replace tab bar with minimal status line (pulse + counts + version)
- Maintenance becomes a toggleable left sidebar (m key)
  - Compact list: active (●) and scheduled (○) windows
  - Enter opens full-screen detail with end/delete actions
- Settings becomes a centered floating overlay (S key)
  - Alerts/Nodes/Users sub-sections with left/right navigation
  - Full CRUD support inside overlay with returnState tracking
- Remove currentTab, switchTab, tab constants, tab click zones
- Replace deleteTab with deleteKind string for cleaner dispatch
- Separate settingsCursor/settingsOffset from main cursor
- Panel focus: maint → monitors → logs → detail
- Context-sensitive footer key hints per focused panel

internal/tui/view_settings_overlay.go

@@ -0,0 +1,87 @@
+package tui
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/charmbracelet/lipgloss"
+)
+
+func (m Model) viewSettingsOverlay() string {
+	overlayW := m.termWidth - 8
+	if overlayW > 90 {
+		overlayW = 90
+	}
+	if overlayW < 40 {
+		overlayW = 40
+	}
+
+	overlayH := m.termHeight - 6
+	if overlayH < 10 {
+		overlayH = 10
+	}
+
+	savedCursor, savedOffset := m.cursor, m.tableOffset
+	savedContentW := m.contentWidth
+	m.cursor = m.settingsCursor
+	m.tableOffset = m.settingsOffset
+	m.contentWidth = overlayW - 4
+
+	content := m.viewSettingsTab()
+
+	m.cursor = savedCursor
+	m.tableOffset = savedOffset
+	m.contentWidth = savedContentW
+
+	settingsListLen := m.settingsListLen()
+	footer := m.settingsFooterKeys()
+	if settingsListLen > 0 {
+		footer = fmt.Sprintf("%d items  %s", settingsListLen, footer)
+	}
+	footerLine := m.st.subtleStyle.Render(footer)
+
+	inner := content + "\n" + footerLine
+	box := m.titledPanel("Settings", inner, overlayW, true)
+
+	boxLines := lipgloss.Height(box)
+	if boxLines > overlayH {
+		lines := splitLines(box, overlayH)
+		box = lines
+	}
+
+	return placeOverlay(box, m.termWidth, m.termHeight)
+}
+
+func (m Model) settingsFooterKeys() string {
+	switch m.settingsSection {
+	case sectionAlerts:
+		return "[n]New [e]Edit [i]Info [d]Del [t]Test [←/→]Section [Esc]Close"
+	case sectionUsers:
+		if m.isAdmin {
+			return "[n]Add [d]Revoke [←/→]Section [Esc]Close"
+		}
+		return "[←/→]Section [Esc]Close"
+	default:
+		return "[←/→]Section [Esc]Close"
+	}
+}
+
+func (m Model) settingsListLen() int {
+	switch m.settingsSection {
+	case sectionAlerts:
+		return len(m.alerts)
+	case sectionNodes:
+		return len(m.nodes)
+	case sectionUsers:
+		return len(m.users)
+	}
+	return 0
+}
+
+func splitLines(s string, maxLines int) string {
+	lines := strings.Split(s, "\n")
+	if len(lines) > maxLines {
+		lines = lines[:maxLines]
+	}
+	return strings.Join(lines, "\n")
+}