Period comparison broken (toggle no-op + wrong-shape consumer)

Severity: High · Where: ControlsBar.stx:43-48, InsightsTab.stx:40-43 vs stats.ts:1096-1110 window.toggleComparison is never assigned (main-dashboard Compare button is a no-op); InsightsTab binds to fields /comparison doesn't return → all four cards render zero. Fix: Wire toggleComparison; map the endpoint's current/previous/changes shape in InsightsTab.