fix(tui): vertically center sparse tab content

Tables on tabs with few rows (Alerts, Nodes, Maint, Users) now sit in
the upper third of the viewport instead of flush against the tab bar.
Dense tabs like Monitors and Logs fill naturally and are unaffected.

internal/tui/view_dashboard.go

@@ -181,7 +181,18 @@ func (m Model) viewDashboard() string {
 	if contentHeight < 1 {
 		contentHeight = 1
 	}
-	paddedContent := lipgloss.NewStyle().Height(contentHeight).MaxHeight(contentHeight).Render(content)
+
+	contentLines := lipgloss.Height(content)
+	var paddedContent string
+	if contentLines < contentHeight {
+		topPad := (contentHeight - contentLines) / 3
+		paddedContent = lipgloss.NewStyle().
+			PaddingTop(topPad).
+			Height(contentHeight).MaxHeight(contentHeight).
+			Render(content)
+	} else {
+		paddedContent = lipgloss.NewStyle().Height(contentHeight).MaxHeight(contentHeight).Render(content)
+	}
 
 	return outerPad.Render(lipgloss.JoinVertical(lipgloss.Top, header, paddedContent, footer))
 }