fix(tui): correct delete confirm labels, normalize help text

Delete confirmation dialog used wrong magic numbers for entity kind
mapping — maintenance windows showed as "alert", settings deletions
showed as "monitor". Now uses tab/section constants to match correctly.

Also: consistent [q/Esc] Back across all views, extract detailKeys(),
remove dead maxSections variable, add nodes empty state hint, use
models.Status constants in siteOrder, delete orphaned theme.png.

internal/tui/tab_nodes.go

@@ -8,7 +8,7 @@ import (
 
 func (m Model) viewNodesTab() string {
 	if len(m.nodes) == 0 {
-		return m.emptyState("No probe nodes connected.", "")
+		return m.emptyState("No probe nodes connected.", "Probes auto-register on connection.")
 	}
 
 	var headers []string