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/styles/app.css

@@ -172,7 +172,14 @@ a { color: inherit; text-decoration: none; }
 .bottom-nav-link:hover { color: var(--text-2); }
 
 @media (max-width: 640px) {
+  /* On mobile the bottom bar is the single primary navigation system. The top
+     bar's Primary links are hidden so the two are never both shown; the top bar
+     keeps only the logo and the Admin operator utility. */
   .app-bottom-nav { display: flex; }
+  .app-nav .nav-links { display: none; }
+  .app-nav { justify-content: space-between; }
+  /* Keep page content clear of the fixed bottom bar. */
+  .wk-page { padding-bottom: calc(60px + env(safe-area-inset-bottom, 0) + var(--s4)); }
 }
 
 /* ── Focus visibility (accessibility) ── */