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/api.ts

@@ -116,17 +116,12 @@ export async function fetchWeekend(meetingKey: number): Promise<Weekend> {
 }
 
 // fetchWeekendContext consumes the canonical /api/v1/weekend-context endpoint
-// (sibling backend story #72). It resolves to null when the endpoint is not yet
-// available (404 / server without the handler) so the Weekend page can fall back
-// to client-side derivation from existing endpoints. Any other HTTP error throws.
-export async function fetchWeekendContext(): Promise<WeekendContext | null> {
-  let res: Response
-  try {
-    res = await fetch('/api/v1/weekend-context')
-  } catch {
-    return null
-  }
-  if (res.status === 404) return null
+// (backend story #72). The response is the authoritative WeekendContext shape and
+// is used verbatim as the Weekend home's source of truth. Any HTTP error throws
+// so the hook can surface an explicit error state; there is no client-side
+// re-derivation of the contract.
+export async function fetchWeekendContext(): Promise<WeekendContext> {
+  const res = await fetch('/api/v1/weekend-context')
   if (!res.ok) {
     throw new Error(`API ${res.status}: ${res.statusText}`)
   }