refactor(dashboard): retire the legacy opt-in hero

The digest-driven Today hero is the single promoted hero at the top of
the dashboard, so the old opt-in hero band is redundant — a user who had
opted in and has Insights on saw two heroes. Remove the legacy hero and
its heroVisible layout flag.

- Drop the DashboardHero / DashboardHeroSkeleton mount from the
  dashboard page; the header now renders the greeting unconditionally
  (its showGreeting gate is gone with the hero that owned it).
- Retire the heroVisible field from the dashboard layout (type,
  defaults, resolver, serializer) and the widgets PUT schema. The
  resolver is a whitelist constructor, so a stored heroVisible is
  dropped on read and never re-persisted — no stored layout breaks.
- Delete the now-orphaned hero components (band, skeleton, ring
  carousel, briefing spotlight) and their tests; drop the retired
  dashboard.hero copy across all locales.
- Remove the hero-visibility toggle from Settings → Dashboard. The
  score-ring picker no longer hangs off that flag and stays available,
  since the rings it configures still ride the server-computed
  dashboard snapshot.
- Repoint the phone-width density guard at the Today hero and mock the
  daily digest so it renders.
- Repoint the preserve-when-absent doc analogy off the removed field.

e2e/utils/mock-dashboard-snapshot.ts

@@ -231,11 +231,9 @@ export interface MockSnapshotOptions {
   summaries?: Record<string, DataSummary>;
   /** Artificial latency in ms before the route fulfils (race specs). */
   delayMs?: number;
-  /** Render the dashboard hero band (`layout.heroVisible` opt-in). */
-  heroVisible?: boolean;
-  /** Fresh (`ready`, non-stale) briefing for the hero spotlight rows. */
+  /** Fresh (`ready`, non-stale) briefing for the dashboard briefing card. */
   briefing?: DailyBriefing;
-  /** Selected hero score rings (renders beside the health-score ring). */
+  /** Selected score rings carried on the snapshot (server-computed). */
   scoreRings?: DashboardScoreRing[];
 }
 
@@ -263,9 +261,7 @@ export function buildMockSnapshot(
       onboardingTourCompleted: true,
       greetingHour: 9,
     },
-    layout: options.heroVisible
-      ? { ...DEFAULT_DASHBOARD_LAYOUT, heroVisible: true }
-      : DEFAULT_DASHBOARD_LAYOUT,
+    layout: DEFAULT_DASHBOARD_LAYOUT,
     layoutCatalogue: buildLayoutCatalogue(),
     metricStates: {},
     targetBands: mockTargetBands(),