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_settings.go

@@ -0,0 +1,58 @@
+package tui
+
+import (
+	"github.com/charmbracelet/lipgloss"
+)
+
+func (m Model) viewSettingsTab() string {
+	maxSections := 2
+	if m.isAdmin {
+		maxSections = 3
+	}
+
+	sections := []string{"Alerts", "Nodes"}
+	if m.isAdmin {
+		sections = append(sections, "Users")
+	}
+	_ = maxSections
+
+	var tabs []string
+	for i, name := range sections {
+		if i == m.settingsSection {
+			tabs = append(tabs, m.st.activeTab.Render(name))
+		} else {
+			tabs = append(tabs, m.st.inactiveTab.Render(name))
+		}
+	}
+	header := lipgloss.JoinHorizontal(lipgloss.Top, tabs...)
+
+	var content string
+	switch m.settingsSection {
+	case sectionAlerts:
+		content = m.viewAlertsTab()
+	case sectionNodes:
+		content = m.viewNodesTab()
+	case sectionUsers:
+		if m.isAdmin {
+			content = m.viewUsersTab()
+		}
+	}
+
+	return header + "\n" + content
+}
+
+func (m *Model) switchSettingsSection(idx int) {
+	max := 1
+	if m.isAdmin {
+		max = 2
+	}
+	if idx > max {
+		idx = 0
+	}
+	if idx < 0 {
+		idx = max
+	}
+	m.settingsSection = idx
+	m.cursor = 0
+	m.tableOffset = 0
+}