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.

messages/de.json

@@ -3240,6 +3240,7 @@
       },
       "vitals": {
         "sectionTitle": "Deine Vitalwerte",
+        "sectionSubtitle": "Ruhewerte von Herz-Kreislauf und Atmung im Vergleich zu deiner eigenen Baseline",
         "loadingLabel": "Vitalwerte werden geladen",
         "loadError": "Deine Vitalwerte konnten gerade nicht geladen werden.",
         "building": "Typischer Bereich entsteht — {count} von {target} Tagen",
@@ -3366,6 +3367,7 @@
       },
       "scores": {
         "sectionTitle": "Deine Gesundheitswerte",
+        "sectionSubtitle": "Zusammengesetzte Werte für Erholung, Stress und Belastung aus deinen verbundenen Geräten",
         "loadError": "Deine Wellness-Werte konnten gerade nicht geladen werden.",
         "recovery": "Erholung",
         "stress": "Stress",

messages/en.json

@@ -3240,6 +3240,7 @@
       },
       "vitals": {
         "sectionTitle": "Your vitals",
+        "sectionSubtitle": "Resting cardiovascular and respiratory signals against your own baseline",
         "loadingLabel": "Loading your vitals",
         "loadError": "Could not load your vitals right now.",
         "building": "Building your typical range — {count} of {target} days",
@@ -3366,6 +3367,7 @@
       },
       "scores": {
         "sectionTitle": "Your health scores",
+        "sectionSubtitle": "Composite recovery, stress and strain read from your connected devices",
         "loadError": "Could not load your wellness scores right now.",
         "recovery": "Recovery",
         "stress": "Stress",