Combine Rules and Admin corner links into one upper-right menu

Replace the fixed Rules link (upper-left) and the creator-only Admin
link (upper-right) with a single collapsible ☰ Menu in the upper right,
leaving the event log alone in its corner. CornerMenu always lists the
Rules link; pages can contribute context-specific entries through the
extraMenuLinks store, which Dashboard uses for the creator-only Admin
link. This gives future misc sub-pages a home without eating more
screen corners.

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

frontend/src/App.svelte

@@ -4,6 +4,7 @@
     import Join from './pages/Join.svelte';
     import Dashboard from './pages/Dashboard.svelte';
     import Admin from './pages/Admin.svelte';
+    import CornerMenu from './components/CornerMenu.svelte';
 
     const routes = {
         '/': Home,
@@ -15,26 +16,5 @@
 
 <main>
     <Router {routes} />
-    <a class="rules-corner-link" href="/rules" target="_blank" rel="noopener" title="Open the rulebook in a new tab">📖 Rules</a>
+    <CornerMenu />
 </main>
-
-<style>
-    .rules-corner-link {
-        position: fixed;
-        top: 12px;
-        left: 16px;
-        z-index: 1001;
-        padding: 6px 14px;
-        border-radius: 16px;
-        background: rgba(10, 15, 25, 0.9);
-        border: 1px solid var(--glass-border-focus);
-        color: var(--neon-cyan);
-        font-size: 0.85rem;
-        font-weight: 700;
-        text-decoration: none;
-        font-family: var(--font-heading);
-    }
-    .rules-corner-link:hover {
-        background: rgba(0, 242, 254, 0.1);
-    }
-</style>