fix: resolve 9 MC bugs in HumanFightPage and useHumanChallenge

- BUG-1: Prevent double-tap by locking phase before async submit
- BUG-2: Submit timeout notification to server when timer expires
- BUG-3: Distinct "TIME'S UP!" visual vs "ANSWER SUBMITTED"
- BUG-4: Track consecutive poll failures, show connection lost banner
- BUG-5: Add A-D / 1-4 keyboard shortcuts for MC choices
- BUG-6: Use choice text as v-for key instead of array index
- BUG-7: Deadline-based timer (250ms tick) prevents drift
- BUG-8: Validate choice is in current choices before submit
- BUG-9: Submit empty timeout instead of random choice on expiry

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

frontend/src/composables/useHumanChallenge.ts

@@ -48,10 +48,9 @@ export function useHumanChallenge(
       if (humanTimer.value <= 0) {
         if (timerHandle) clearInterval(timerHandle)
         if (!humanSubmitted.value) {
-          if (humanChoices.value.length > 0 && !humanAnswer.value.trim()) {
-            humanAnswer.value = humanChoices.value[Math.floor(Math.random() * humanChoices.value.length)]
-          }
-          void submitHumanAnswer()
+          // BUG-9: Submit empty timeout instead of random choice
+          humanSubmitted.value = true
+          void submitTimeout()
         }
       }
     }, 1000)
@@ -76,10 +75,25 @@ export function useHumanChallenge(
   }
 
   function submitChoice(choice: string) {
+    if (humanSubmitted.value) return // Prevent double-tap
     humanAnswer.value = choice
+    humanSubmitted.value = true // Lock immediately before async
     void submitHumanAnswer()
   }
 
+  async function submitTimeout() {
+    if (!myBotId.value || !humanChallenge.value) return
+    try {
+      await fetch(`/api/fights/${fightId.value}/respond/${myBotId.value}`, {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({ answer: '', timeout: true }),
+      })
+    } catch {
+      // Server will time out on its own
+    }
+  }
+
   async function pollForChallenge() {
     if (!myBotId.value) return
     try {