feat(tui): consolidate 6 tabs to 3, add log sidebar

Tab bar: Monitors | Maint | Settings (was 6 tabs).

Settings tab merges Alerts, Nodes, Users as sub-sections with
left/right arrow navigation. Each section keeps its own cursor,
keybindings, and CRUD operations.

Monitors tab now shows a log sidebar at >= 120 cols (70/30 split).
Under 120 cols, monitors render full-width without logs.

- Introduced tab constants (tabMonitors, tabMaint, tabSettings)
- Introduced section constants (sectionAlerts, sectionNodes, sectionUsers)
- Removed stateLogs and stateUsers states
- All magic tab numbers replaced with named constants

internal/tui/data.go

@@ -106,7 +106,7 @@ func (m *Model) refreshLive() {
 	m.sites = ordered
 	m.refreshLogContent()
 
-	if m.currentTab == 0 && m.selectedID != 0 {
+	if m.currentTab == tabMonitors && m.selectedID != 0 {
 		for i, s := range m.sites {
 			if s.ID == m.selectedID {
 				m.cursor = i
@@ -118,7 +118,7 @@ func (m *Model) refreshLive() {
 }
 
 func (m *Model) syncSelectedID() {
-	if m.currentTab == 0 && m.cursor < len(m.sites) {
+	if m.currentTab == tabMonitors && m.cursor < len(m.sites) {
 		m.selectedID = m.sites[m.cursor].ID
 	}
 }