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/tab_users.go

@@ -116,7 +116,12 @@ func (m *Model) submitUserForm() tea.Cmd {
 	ctx := m.ctx
 	id := m.editID
 	username, key, role := d.Username, d.PublicKey, d.Role
-	m.state = stateDashboard
+	if m.returnState == stateSettings {
+		m.state = stateSettings
+	} else {
+		m.state = stateDashboard
+	}
+	m.returnState = 0
 	if id > 0 {
 		return writeCmd("Update user", func() error {
 			return st.UpdateUser(ctx, id, username, key, role)