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"