Add a subtitle slot to the insights overview section heading

Detail pages already render a one-line description under their heading;
the overview section headings had no slot for one. Add an optional
subtitle to SectionHeading, rendered in the same muted typography, and
wire it into the obvious top-level sections: Trends (folding its existing
right-aligned caption into the slot), Your health scores, and Your
vitals.

src/components/insights/derived/vitals-dashboard.tsx

@@ -729,6 +729,7 @@ export function VitalsDashboard({ batch, layout, className }: DashboardProps) {
           <SectionHeading
             icon={HeartPulse}
             title={t("insights.derived.vitals.sectionTitle")}
+            subtitle={t("insights.derived.vitals.sectionSubtitle")}
           />
           <div
             data-slot="vitals-dashboard-error"
@@ -774,6 +775,7 @@ export function VitalsDashboard({ batch, layout, className }: DashboardProps) {
           <SectionHeading
             icon={HeartPulse}
             title={t("insights.derived.vitals.sectionTitle")}
+            subtitle={t("insights.derived.vitals.sectionSubtitle")}
           />
           <div
             data-slot="vitals-dashboard-grid"

src/components/insights/derived/wellness-scores.tsx

@@ -211,6 +211,7 @@ export function WellnessScores({
         <SectionHeading
           icon={Activity}
           title={t("insights.derived.scores.sectionTitle")}
+          subtitle={t("insights.derived.scores.sectionSubtitle")}
         />
         <div
           data-slot="wellness-scores-error"
@@ -394,6 +395,7 @@ export function WellnessScores({
       <SectionHeading
         icon={Activity}
         title={t("insights.derived.scores.sectionTitle")}
+        subtitle={t("insights.derived.scores.sectionSubtitle")}
       />
       <div
         data-slot="wellness-scores-grid"

src/components/insights/section-heading.tsx

@@ -34,6 +34,14 @@ interface SectionHeadingProps {
   icon: ComponentType<{ className?: string }>;
   /** Heading text rendered inside the `<h2>`. */
   title: ReactNode;
+  /**
+   * Optional one-line description rendered directly under the `<h2>`, in the
+   * same muted typography the detail pages use beneath their headings
+   * (`text-muted-foreground text-sm`). Opt-in per section so the overview can
+   * carry the "descriptive line under every heading" rule without forcing a
+   * subtitle where none reads well.
+   */
+  subtitle?: ReactNode;
   /** Optional trailing affordance pinned to the right edge of the row. */
   action?: ReactNode;
   /** Optional id on the `<h2>`, e.g. for an `aria-labelledby` link. */
@@ -44,6 +52,7 @@ interface SectionHeadingProps {
 export function SectionHeading({
   icon: Icon,
   title,
+  subtitle,
   action,
   id,
   className,
@@ -58,9 +67,19 @@ export function SectionHeading({
     >
       <div className="flex min-w-0 items-center gap-2">
         <Icon className="text-foreground h-5 w-5 shrink-0" aria-hidden="true" />
-        <h2 id={id} className="text-base font-semibold">
-          {title}
-        </h2>
+        <div className="min-w-0">
+          <h2 id={id} className="text-base font-semibold">
+            {title}
+          </h2>
+          {subtitle ? (
+            <p
+              data-slot="section-heading-subtitle"
+              className="text-muted-foreground text-sm"
+            >
+              {subtitle}
+            </p>
+          ) : null}
+        </div>
       </div>
       {action ? (
         <div className="flex shrink-0 items-center gap-2">{action}</div>

src/components/insights/trends-row.tsx

@@ -200,11 +200,7 @@ export function TrendsRow({
       <SectionHeading
         icon={TrendingUp}
         title={t("insights.trendsRow.title")}
-        action={
-          <p className="text-muted-foreground text-xs">
-            {t("insights.trendsRow.subtitle")}
-          </p>
-        }
+        subtitle={t("insights.trendsRow.subtitle")}
       />
       {/* v1.4.22 A4 — equal-height cards. The annotation prose
           below each chart varies in length, which used to leave the