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?"

src/components/insights/sub-page-shell.tsx

@@ -168,7 +168,7 @@ export function SubPageShell({
     // state + renders the sheet, so the gear and the card don't have to
     // share a parent beyond this shell.
     <TargetAdjustProvider>
-      <div data-slot="insights-subpage" className="space-y-4 md:space-y-5">
+      <div data-slot="insights-subpage" className="space-y-3 md:space-y-4">
         {/* v1.8.7.1 — back-nav leads the page, above the heading. */}
         {backLink}
         <header className="space-y-1.5">
@@ -262,13 +262,11 @@ export function SubPageShell({
           brand-new-metric paths. Numbers-first is the Apple-Health /
           Withings / Oura detail-screen convention.
 
-          The stats describe the full range. Recomputing them for a window
-          the user brushes / selects in the chart below would mean lifting the
-          chart's domain into this shell so both the chart and the strip read
-          it — but the shared `<HealthChart>` (also driving the dashboard) has
-          no Recharts `<Brush>` or domain-change callback and keeps its range
-          state internal, so that wiring is left for a later release to avoid
-          destabilising the chart. */}
+          v1.12.7 — the stats are chart-reactive: when the user brushes a
+          window in the chart below, the page lifts the chart's per-type
+          windowed Min / Max / Median / Mean and threads it back into the
+          strip, which swaps the header to a "selected range" pill. No
+          selection falls back to the cheap full-range summary. */}
         {statStrip}
         {/* v1.12.6 — the canonical spine body: intro (header) → stat strip
           (above) → chart → target card → assessment. The page renders

src/components/insights/vo2-max-chart-row.tsx

@@ -193,7 +193,13 @@ export function Vo2MaxChartRow({
 
       {/* Trend chart. The chartKey isolates per-chart overlay prefs from
           the dashboard VO2 tile and from the pulse chart sitting above
-          it on the same page. */}
+          it on the same page.
+
+          The chart mounts as soon as there is at least one reading. VO₂ max
+          arrives from Apple Health at most every few days, so the series is
+          inherently sparse; the line carries a visible dot per point
+          (`dot r=3`), so a one- or two-sample series still reads as a real
+          chart. A zero-reading account keeps the short onboarding note. */}
       {hasData ? (
         <HealthChartDynamic
           chartKey="vo2Max"