feat(web): set up React app shell with routing and layout

Adds react-router-dom with routes for ship list, dashboard, and rules pages.
Includes TopBar with navigation and mobile-first dark theme CSS.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/src/components/layout/PageContainer.tsx

@@ -0,0 +1,5 @@
+import type { ReactNode } from 'react';
+
+export default function PageContainer({ children }: { children: ReactNode }) {
+  return <main className="page-container">{children}</main>;
+}

web/src/components/layout/TopBar.tsx

@@ -0,0 +1,32 @@
+import { useNavigate, useLocation } from 'react-router-dom';
+
+export default function TopBar({ title }: { title?: string }) {
+  const navigate = useNavigate();
+  const location = useLocation();
+  const isHome = location.pathname === '/';
+
+  return (
+    <header className="top-bar">
+      <div className="top-bar-left">
+        {!isHome && (
+          <button className="top-bar-back" onClick={() => navigate(-1)}>
+            &larr;
+          </button>
+        )}
+        <h1 className="top-bar-title">{title || 'Spelljammer'}</h1>
+      </div>
+      <nav className="top-bar-right">
+        {location.pathname !== '/rules' && (
+          <button className="top-bar-link" onClick={() => navigate('/rules')}>
+            Rules
+          </button>
+        )}
+        {!isHome && location.pathname !== '/rules' && (
+          <button className="top-bar-link" onClick={() => navigate('/')}>
+            Ships
+          </button>
+        )}
+      </nav>
+    </header>
+  );
+}