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.

docs/api/openapi.yaml

@@ -11535,8 +11535,8 @@ components:
           const: 1
         view:
           description: Which presentation /medications renders in. Default "cards". Optional on PUT — when omitted the stored
-            value is preserved (preserve-when-absent, like `heroVisible` on the dashboard layout). Always present on
-            responses.
+            value is preserved (preserve-when-absent, like `chartOverlayPrefs` on the dashboard layout). Always present
+            on responses.
           type: string
           enum:
             - cards
@@ -30064,8 +30064,8 @@ components:
           const: 1
         view:
           description: Which presentation /medications renders in. Default "cards". Optional on PUT — when omitted the stored
-            value is preserved (preserve-when-absent, like `heroVisible` on the dashboard layout). Always present on
-            responses.
+            value is preserved (preserve-when-absent, like `chartOverlayPrefs` on the dashboard layout). Always present
+            on responses.
           type: string
           enum:
             - cards