fix: mobile — scale sprites to canvas size, fix viewport scroll

- Add scale factor (SF) based on canvas height so bots and referee
  shrink proportionally on small screens (reference: 500px)
- Scale judge chair/ladder dimensions with SF
- Push judge seat lower on small canvases so it stays visible
- Use 100dvh instead of 100vh to account for mobile browser chrome
  (address bar, tab bar) — eliminates scroll under browser UI
- App root uses dvh, main area uses min-h-0 + overflow-y-auto

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

frontend/src/App.vue

@@ -4,10 +4,10 @@ import NavBar from './components/NavBar.vue'
 </script>
 
 <template>
-  <div class="min-h-screen bg-surface synthwave-grid flex flex-col relative">
+  <div class="h-[100dvh] bg-surface synthwave-grid flex flex-col relative">
     <div class="fixed inset-0 crt-overlay z-40" />
     <NavBar />
-    <main class="flex-1 relative z-10">
+    <main class="flex-1 min-h-0 relative z-10 overflow-y-auto">
       <RouterView />
     </main>
   </div>