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>

tests/visual/mvp-screens.spec.ts

@@ -1,6 +1,6 @@
 import { test } from '@playwright/test'
 import {
-  gotoCommandCenterReady,
+  gotoWeekendReady,
   gotoDataLibraryReady,
   gotoLiveEmptyReady,
   gotoRaceHubReady,
@@ -8,9 +8,9 @@ import {
 } from './helpers'
 
 test.describe('MVP visual regression', () => {
-  test('command-center', async ({ page }) => {
-    await gotoCommandCenterReady(page)
-    await screenshotPage(page, 'command-center')
+  test('weekend', async ({ page }) => {
+    await gotoWeekendReady(page)
+    await screenshotPage(page, 'weekend')
   })
 
   test('race-hub', async ({ page }) => {