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/update_test.go

@@ -266,7 +266,7 @@ func TestConfirmDelete_WritesOffUIGoroutine(t *testing.T) {
 	m := newTestModel(ms)
 	m.sites = []models.Site{{SiteConfig: models.SiteConfig{ID: 4, Name: "s"}}}
 	m.state = stateConfirmDelete
-	m.deleteTab = 0
+	m.deleteKind = "site"
 	m.deleteID = 4
 
 	updated, cmd := (&m).handleConfirmDelete(keyMsg("y"))