feat(web): move mod header into site header

Replace the global TTS Workshop header on mod routes with a ModHeader showing the thumbnail, game name, and download/setup links styled like the Search/BGM nav.

apps/web/src/App.tsx

@@ -1,7 +1,8 @@
 import { lazy, Suspense } from 'react';
-import { Link, Route, Routes } from 'react-router-dom';
+import { Link, Route, Routes, useLocation } from 'react-router-dom';
 import SearchPage from './pages/SearchPage';
 import ModPage from './pages/ModPage';
+import ModHeader from './components/ModHeader';
 
 // The full-setup view and the bgm/tabletop pages pull in the whole three.js
 // stack (Scene + all mesh viewers + @tts/tabletop). Lazy-load them so that
@@ -18,22 +19,29 @@ const SetupsPage = lazy(() => import('./pages/SetupsPage'));
 const SetupPage = lazy(() => import('./pages/SetupPage'));
 
 export default function App() {
+  const { pathname } = useLocation();
+  const isModRoute = pathname.startsWith('/mod/');
+
   return (
     <div className="min-h-screen bg-zinc-950 text-zinc-100">
       <header className="border-b border-zinc-800">
-        <div className="mx-auto flex max-w-5xl items-center justify-between px-4 py-4">
-          <Link to="/" className="text-lg font-semibold tracking-tight">
-            TTS Workshop
-          </Link>
-          <nav className="flex gap-4 text-sm text-zinc-400">
-            <Link to="/" className="hover:text-zinc-100">
-              Search
+        {isModRoute ? (
+          <ModHeader />
+        ) : (
+          <div className="mx-auto flex max-w-5xl items-center justify-between px-4 py-4">
+            <Link to="/" className="text-lg font-semibold tracking-tight">
+              TTS Workshop
             </Link>
-            <Link to="/bgm" className="hover:text-zinc-100">
-              BGM
-            </Link>
-          </nav>
-        </div>
+            <nav className="flex gap-4 text-sm text-zinc-400">
+              <Link to="/" className="hover:text-zinc-100">
+                Search
+              </Link>
+              <Link to="/bgm" className="hover:text-zinc-100">
+                BGM
+              </Link>
+            </nav>
+          </div>
+        )}
       </header>
       <main className="mx-auto max-w-5xl px-4 py-8">
         <Routes>