feat: ranked queue Elo-bracket matchmaking and harder prompts

Elo-bracket matchmaking: prefer ±200, widen by 100 every 15s of waiting.
Add pickRankedChallenge() that filters to creative/open-ended only, never
multiple choice. Show ranked queue status, wait estimate, and "HARDER
PROMPTS" notice on JoinBoutPage.

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

frontend/src/pages/JoinBoutPage.vue

@@ -25,6 +25,8 @@ const isJoining = ref(false)
 const isJoiningRanked = ref(false)
 const isJoiningPractice = ref(false)
 const queueCount = ref(0)
+const rankedQueueCount = ref(0)
+const rankedEstWait = ref(15)
 let pollHandle: ReturnType<typeof setInterval> | null = null
 
 // Registration form
@@ -112,11 +114,19 @@ onUnmounted(() => {
 
 async function pollQueue() {
   try {
-    const res = await fetch('/api/queue/status')
-    if (res.ok) {
-      const data = await res.json()
+    const [freeRes, rankedRes] = await Promise.all([
+      fetch('/api/queue/status'),
+      fetch('/api/queue/ranked-status'),
+    ])
+    if (freeRes.ok) {
+      const data = await freeRes.json()
       queueCount.value = data.waiting
     }
+    if (rankedRes.ok) {
+      const data = await rankedRes.json()
+      rankedQueueCount.value = data.waiting
+      rankedEstWait.value = data.estimatedWaitSec ?? 15
+    }
   } catch (err) {
     console.warn('[JoinBout] queue poll failed:', err)
   }
@@ -1039,6 +1049,15 @@ function handleSignOut() {
             <span class="text-[9px] font-mono tracking-normal font-normal text-neon-cyan/70">21 SATS — WINNER TAKES ALL</span>
           </button>
 
+          <!-- Ranked queue info -->
+          <div v-if="!isHumanMode && !bot?.isHuman" class="flex items-center justify-between px-1">
+            <span class="font-mono text-[9px] text-text-muted">
+              {{ rankedQueueCount > 0 ? `${rankedQueueCount} waiting` : 'No queue' }}
+              — ~{{ rankedEstWait }}s wait
+            </span>
+            <span class="font-pixel text-[8px] text-neon-purple tracking-wider">HARDER PROMPTS</span>
+          </div>
+
           <!-- Wallet connect (shown if no wallet) -->
           <WalletConnect v-if="!isHumanMode && !bot.isHuman" />