refactor(tui): consistent chrome across all views

- Extract divider() and emptyState() helpers to format.go
- All empty states now use bordered box with accent color
- Detail and alert detail panels get header/section dividers
- SLA label width 14→16 to match detail/alert panels
- Logs key hints moved from content to dashboard footer
- History/SLA panels use shared divider helper

internal/tui/tab_nodes.go

@@ -7,7 +7,7 @@ import (
 
 func (m Model) viewNodesTab() string {
 	if len(m.nodes) == 0 {
-		return "\n  No probe nodes connected."
+		return m.emptyState("No probe nodes connected.", "")
 	}
 
 	var headers []string