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/components/Nav.tsx

@@ -8,14 +8,23 @@ const PRIMARY = [
   { to: '/explore', label: 'Explore', icon: Compass, exact: false },
 ] as const
 
+/**
+ * Nav renders one primary navigation system per breakpoint:
+ *   - Desktop/tablet: the top bar's `aria-label="Primary"` links.
+ *   - Mobile (≤640px): the bottom `aria-label="Primary"` bar; the top bar's links
+ *     are hidden via CSS so the two are never both active at once.
+ *
+ * Admin is an operator utility, deliberately outside every Primary landmark — it
+ * lives in a plain toolbar slot and never appears in the mobile bottom nav.
+ */
 export function Nav() {
   return (
     <>
-      <nav className="app-nav" aria-label="Primary">
+      <header className="app-nav">
         <Link to="/" className="nav-logo">
           box<em>-</em>box
         </Link>
-        <div className="nav-links">
+        <nav className="nav-links" aria-label="Primary">
           {PRIMARY.map(({ to, label, exact }) => (
             <Link
               key={to}
@@ -26,8 +35,8 @@ export function Nav() {
               {label}
             </Link>
           ))}
-        </div>
-        <div className="nav-utility">
+        </nav>
+        <div className="nav-utility" role="toolbar" aria-label="Operator utilities">
           <Link
             to="/admin"
             className="nav-utility-link"
@@ -36,9 +45,9 @@ export function Nav() {
             Admin
           </Link>
         </div>
-      </nav>
+      </header>
 
-      <nav className="app-bottom-nav" aria-label="Primary mobile">
+      <nav className="app-bottom-nav" aria-label="Primary">
         {PRIMARY.map(({ to, label, icon: Icon, exact }) => (
           <Link
             key={to}