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/data.go

@@ -125,7 +125,7 @@ func (m *Model) refreshLive() {
 	m.sites = ordered
 	m.refreshLogContent()
 
-	if m.currentTab == tabMonitors && m.selectedID != 0 {
+	if m.selectedID != 0 {
 		for i, s := range m.sites {
 			if s.ID == m.selectedID {
 				m.cursor = i
@@ -137,7 +137,7 @@ func (m *Model) refreshLive() {
 }
 
 func (m *Model) syncSelectedID() {
-	if m.currentTab == tabMonitors && m.cursor < len(m.sites) {
+	if m.cursor < len(m.sites) {
 		m.selectedID = m.sites[m.cursor].ID
 	}
 }