Add HTML rulebook page with always-available Rules link

New /#/rules route renders the full rulebook transcribed from
"Rats with Gats beta.pdf" (intro, character creation, gameplay,
obstacle charts, challenge resolution, example of play, and setting
info), with an in-page table of contents. A fixed corner link in
App.svelte opens it in a new tab from every page, and the splash
page's previously broken /rules link now points at the new route.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/App.svelte

@@ -4,15 +4,18 @@
     import Join from './pages/Join.svelte';
     import Dashboard from './pages/Dashboard.svelte';
     import Admin from './pages/Admin.svelte';
+    import Rules from './pages/Rules.svelte';
 
     const routes = {
         '/': Home,
         '/game/:id/join': Join,
         '/game/:id/player/:pid': Dashboard,
         '/game/:id/admin': Admin,
+        '/rules': Rules,
     };
 </script>
 
 <main>
     <Router {routes} />
+    <a class="rules-corner-link" href="#/rules" target="_blank" rel="noopener" title="Open the rulebook in a new tab">📖 Rules</a>
 </main>