feat(dashboard): VO2 max trend tile (opt-in, secondary-metric pattern)

Adds the VO2 max strip tile to /, matching the existing
secondary-metric convention (sleep / steps / glucose / oxygen
saturation are all default-invisible + opt-in via Settings →
Dashboard).

Wire-up:
  - src/lib/dashboard-layout.ts: vo2Max appended to
    DASHBOARD_WIDGET_IDS and DEFAULT_DASHBOARD_LAYOUT.widgets with
    visible=false / tileVisible=false. The /api/dashboard/widgets
    Zod enum is derived from the same array so the toggle is wired
    automatically.
  - src/app/page.tsx: vo2Summary from data?.summaries?.VO2_MAX
    (auto-populated by /api/analytics; the route iterates the full
    measurementTypeEnum.options list, no backend change needed),
    hasVo2 data-floor gate, showVo2Tile combined gate, and the
    TrendCard push next to showStepsTile. directionSentiment is
    up-good (higher VO2 max is better) and the unit string runs
    through the t() helper so the iOS app session can localise it.
  - src/components/settings/dashboard-layout-section.tsx: vo2Max
    label key added to WIDGET_LABEL_KEYS so the Settings → Dashboard
    toggle row renders the localized label.
  - messages/{en,de,fr,es,it,pl}.json: dashboard.vo2Max + vo2MaxUnit
    keys. "VO₂ max" is universal across all six locales (scientific
    abbreviation, no translation needed); the unit string is
    consistent SI notation.

No chart-row card — that's an explicit v1.5 deferral per the W8d
outline §6.4. The tile lights up the moment the user accumulates any
VO2 sample (Withings ingest or the upcoming iOS Health passthrough).

src/components/settings/dashboard-layout-section.tsx

@@ -48,6 +48,8 @@ const WIDGET_LABEL_KEYS: Record<DashboardWidgetId, string> = {
   // for the polished `<InsightsCardPreview>` (top severity-ordered AI
   // recommendations + ring confidence meter + "View all" CTA).
   insightsPreview: "dashboard.insightsPreview",
+  // v1.4.25 W8d — VO2 max secondary-metric tile (opt-in).
+  vo2Max: "dashboard.vo2Max",
 };
 
 export function DashboardLayoutSection({ id }: { id: string }) {