refactor(insights): tighten the subpage spine and clarify the VO2 max chart

Pull the subpage spine to a denser space-y-3/4 rhythm to match the tighter
stat strip, and refresh the stale brush comment now that the windowed stats
are wired. Give the cardio-fitness page its Gauge stat-strip glyph so VO2
max leads with the same standard treatment as the other HealthKit metrics;
the dedicated subpage already renders the full chart. Clarify the VO2 row's
sparse-series note.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app/insights/cardio-fitness/page.tsx

@@ -24,6 +24,7 @@ export default function InsightsCardioFitnessPage() {
       color="#bd93f9"
       unit="mL/(kg·min)"
       yAxisUnit="mL/(kg·min)"
+      statIcon={Gauge}
       emptyStateIcon={<Gauge className="size-6" />}
       emptyStateCtaType={null}
       coachPrefill="I haven't logged any cardio-fitness (VO₂ max) data yet — what does this metric tell me about my health, and how do I improve it?"