feat(#73): Adaptive Weekend shell and responsive navigation

Implemented by claude via .agents/dev dispatch.

frontend/src/components/Nav.tsx

@@ -1,36 +1,57 @@
 import { Link } from '@tanstack/react-router'
+import { Compass, FileText, Home, Trophy } from 'lucide-react'
+
+const PRIMARY = [
+  { to: '/', label: 'Weekend', icon: Home, exact: true },
+  { to: '/championship', label: 'Championship', icon: Trophy, exact: false },
+  { to: '/briefing', label: 'Briefing', icon: FileText, exact: false },
+  { to: '/explore', label: 'Explore', icon: Compass, exact: false },
+] as const
 
 export function Nav() {
   return (
-    <nav className="app-nav">
-      <Link to="/" className="nav-logo">
-        box<em>-</em>box
-      </Link>
-      <div className="nav-links">
-        <Link to="/" activeProps={{ className: 'active' }} activeOptions={{ exact: true }}>
-          Command
+    <>
+      <nav className="app-nav" aria-label="Primary">
+        <Link to="/" className="nav-logo">
+          box<em>-</em>box
         </Link>
-        <Link to="/live" activeProps={{ className: 'active' }}>
-          Live
-        </Link>
-        <Link to="/race-hub" search={{}} activeProps={{ className: 'active' }}>
-          Race Hub
-        </Link>
-        <Link to="/preview" activeProps={{ className: 'active' }}>
-          Preview
-        </Link>
-        <Link to="/championship" activeProps={{ className: 'active' }}>
-          Championship
-        </Link>
-        <Link to="/briefing" activeProps={{ className: 'active' }}>
-          Briefing
-        </Link>
-      </div>
-      <div className="nav-utility">
-        <Link to="/admin" className="nav-utility-link" activeProps={{ className: 'nav-utility-link active' }}>
-          Admin
-        </Link>
-      </div>
-    </nav>
+        <div className="nav-links">
+          {PRIMARY.map(({ to, label, exact }) => (
+            <Link
+              key={to}
+              to={to}
+              activeProps={{ className: 'active' }}
+              activeOptions={exact ? { exact: true } : undefined}
+            >
+              {label}
+            </Link>
+          ))}
+        </div>
+        <div className="nav-utility">
+          <Link
+            to="/admin"
+            className="nav-utility-link"
+            activeProps={{ className: 'nav-utility-link active' }}
+          >
+            Admin
+          </Link>
+        </div>
+      </nav>
+
+      <nav className="app-bottom-nav" aria-label="Primary mobile">
+        {PRIMARY.map(({ to, label, icon: Icon, exact }) => (
+          <Link
+            key={to}
+            to={to}
+            className="bottom-nav-link"
+            activeProps={{ className: 'bottom-nav-link active' }}
+            activeOptions={exact ? { exact: true } : undefined}
+          >
+            <Icon size={18} aria-hidden="true" />
+            <span>{label}</span>
+          </Link>
+        ))}
+      </nav>
+    </>
   )
 }