fix(#73): consume canonical Weekend Context contract and repair navigation

Address the independent review blockers on PR #80 after rebasing onto the
authoritative #72 Weekend Context API.

- Replace the invented frontend WeekendContext with the exact backend contract
  (temporal_state, previous/focus/next meetings, previous_completed/active/next/
  default_analysis sessions with availability). Every valid canonical payload now
  maps to a designed state via a total resolveViewState; a well-formed response
  can never fall through to the limited-data placeholder.
- Make the canonical read the single source of truth: useWeekendContext no longer
  fans out to season/meetings/per-weekend/OpenF1/live queries. Only supplementary
  championship + news reads run, and only once the canonical context resolves.
- Fix the Prepare/analysis flow: /preview is a stable alias that renders the
  preparation surface (PreSessionView) instead of redirecting back to the same
  between-races screen.
- One primary navigation system per breakpoint: the mobile top-bar links are
  hidden so the bottom bar is the sole primary nav, and Admin is moved out of
  every Primary landmark into an operator-utilities toolbar.
- Add Vitest coverage for the contract mapping, every temporal state, loading/
  error/limited surfaces, the no-fanout guarantee, the /preview CTA, and the nav
  hierarchy; add hermetic Playwright journeys (seeded + injected canonical
  payloads), 390/768/1440 overflow checks, and Weekend visual snapshots. Retire
  the stale Command Center specs/snapshots.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/router.tsx

@@ -1,4 +1,4 @@
-import { createRootRoute, createRoute, createRouter, Outlet, redirect } from '@tanstack/react-router'
+import { createRootRoute, createRoute, createRouter, Outlet } from '@tanstack/react-router'
 import { Nav } from './components/Nav'
 import { WeekendPage } from './pages/WeekendPage'
 import { ExplorePage } from './pages/ExplorePage'
@@ -97,13 +97,15 @@ export const briefingRoute = createRoute({
   component: BriefingPage,
 })
 
-// Preview folds into the Weekend home. Keep /preview as a stable redirect so saved
-// links resolve into the appropriate Weekend state.
+// Preview folds into the Weekend home. /preview is a stable alias that renders the
+// Weekend page in its preparation surface (PreSessionView) whenever there is a next
+// event — so saved preview links and the "Prepare for …" CTA reach real preview
+// content instead of redirecting back to the same between-races screen.
 export const previewRoute = createRoute({
   getParentRoute: () => rootRoute,
   path: '/preview',
-  beforeLoad: () => {
-    throw redirect({ to: '/' })
+  component: function PreviewRoute() {
+    return <WeekendPage preview />
   },
 })