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/tab_users.go

@@ -116,7 +116,7 @@ func (m *Model) submitUserForm() tea.Cmd {
 	st := m.store
 	id := m.editID
 	username, key, role := d.Username, d.PublicKey, d.Role
-	m.state = stateUsers
+	m.state = stateDashboard
 	if id > 0 {
 		return writeCmd("Update user", func() error {
 			return st.UpdateUser(context.Background(), id, username, key, role)