fix(dashboard): mount InsightsCardPreview on root page (wire B1b dashboard preview to live route)

The polished InsightsCardPreview component shipped in B1b commit 5
(d2cdf9d) was an orphan with zero non-test imports. The dashboard read
no advisor payload, so the top severity-ordered AI recommendation +
ring confidence meter + "View all" CTA was unreachable from any live
route. Wave-D design review flagged this as CRITICAL C2.

Wires the preview into the root dashboard page above the chart row,
sharing the advisor cache key with /insights so a regenerate on either
surface refreshes the other without a second LLM call. The preview
self-hides when the user has no provider configured (route 422 → null)
or the advisor returns zero recommendations.

Adds insightsPreview to DASHBOARD_WIDGET_IDS so the layout toggle
persists through Settings → Dashboard. Pins the widget into
DEFAULT_DASHBOARD_LAYOUT (visible: true, tileVisible: false) so users
discover the feature without hunting through settings, and adds a
contract test mirroring the A5 widget-enum-drift guard.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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

@@ -43,6 +43,10 @@ const WIDGET_LABEL_KEYS: Record<DashboardWidgetId, string> = {
   bpInTarget: "dashboard.bpInTarget",
   oxygenSaturation: "measurements.typeOxygenSaturation",
   achievements: "achievements.title",
+  // v1.4.16 phase D reconcile (CRITICAL C2) — dashboard preview tile
+  // for the polished `<InsightsCardPreview>` (top severity-ordered AI
+  // recommendations + ring confidence meter + "View all" CTA).
+  insightsPreview: "dashboard.insightsPreview",
 };
 
 export function DashboardLayoutSection({ id }: { id: string }) {