refactor(settings): gather layout personalization under one home

Personalizing how the app is laid out was split across four unrelated
settings sections — Dashboard, Insights, Medications, Mood — with no
shared framing, so the same concept (arrange, reorder, show/hide) read
as four disconnected places, plus the settings nav carried four separate
arrange entries.

Add one Layout & Personalization hub that presents the four editors as a
consistent set of linked cards and replaces the four scattered nav
entries with a single home. Each editor keeps its own route and mutation
flow — deep links, the page-header cogs, and the hub's links all still
resolve — and a Layout child route now highlights the hub in the nav.

src/app/settings/[section]/page.tsx

@@ -10,6 +10,7 @@ import { DashboardSection } from "@/components/settings/dashboard-section";
 import { ExportSection } from "@/components/settings/export-section";
 import { InsightsSection } from "@/components/settings/insights-section";
 import { IntegrationsSection } from "@/components/settings/integrations-section";
+import { LayoutSection } from "@/components/settings/layout-section";
 import { MedicationsSection } from "@/components/settings/medications-section";
 import { MoodSection } from "@/components/settings/mood-section";
 import { NotificationsSection } from "@/components/settings/notifications-section";
@@ -51,6 +52,7 @@ const SECTION_COMPONENTS: Record<
   ai: AiSection,
   integrations: IntegrationsSection,
   notifications: NotificationsSection,
+  layout: LayoutSection,
   dashboard: DashboardSection,
   insights: InsightsSection,
   medications: MedicationsSection,