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_sites.go

@@ -101,16 +101,7 @@ func (m Model) computeLayout() tableLayout {
 func (m Model) viewSitesTab() string {
 
 	if len(m.sites) == 0 {
-		welcome := lipgloss.NewStyle().
-			Border(lipgloss.RoundedBorder()).
-			BorderForeground(m.theme.Accent).
-			Padding(1, 3).
-			Render(
-				titleStyle.Render("uptop") + "\n\n" +
-					"No monitors configured yet.\n\n" +
-					subtleStyle.Render("[n] Add your first monitor"),
-			)
-		return "\n" + welcome
+		return m.emptyState(titleStyle.Render("uptop")+"\n\nNo monitors configured yet.", "[n] Add your first monitor")
 	}
 
 	layout := m.computeLayout()