fix: add back-button guard and tab-switch re-sync to HumanFightPage

- Warn before navigating away from active fight (beforeRouteLeave)
- Re-poll challenge state when tab becomes visible (visibilitychange)
- Prevents silent forfeit on back-button and stale timer after tab switch

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

frontend/src/pages/HumanFightPage.vue

@@ -1,6 +1,6 @@
 <script setup lang="ts">
 import { ref, computed, onMounted, onUnmounted, nextTick } from 'vue'
-import { useRoute, useRouter } from 'vue-router'
+import { useRoute, useRouter, onBeforeRouteLeave } from 'vue-router'
 import { useNostr } from '../composables/useNostr'
 import FightViewer from '../components/FightViewer.vue'
 import type { FightData } from '../game/fight/types'
@@ -74,6 +74,16 @@ const hasChoices = computed(() =>
   currentChallenge.value?.choices && currentChallenge.value.choices.length > 0
 )
 
+// Back-button guard — warn if fight is active
+onBeforeRouteLeave((_to, _from, next) => {
+  const isActive = phase.value === 'challenge' || phase.value === 'submitted' || phase.value === 'timeout' || phase.value === 'between' || phase.value === 'waiting'
+  if (isActive && !window.confirm('Fight in progress! Leave and forfeit?')) {
+    next(false)
+    return
+  }
+  next()
+})
+
 onMounted(() => {
   if (!myBot.value) {
     router.push('/join')
@@ -81,6 +91,7 @@ onMounted(() => {
   }
   startPolling()
   window.addEventListener('keydown', handleKeyboard)
+  document.addEventListener('visibilitychange', handleVisibilityChange)
 })
 
 onUnmounted(() => {
@@ -88,8 +99,16 @@ onUnmounted(() => {
   stopTimer()
   if (feedbackTimer.value) clearTimeout(feedbackTimer.value)
   window.removeEventListener('keydown', handleKeyboard)
+  document.removeEventListener('visibilitychange', handleVisibilityChange)
 })
 
+// Tab switching — re-poll and re-sync timer when tab becomes visible
+function handleVisibilityChange() {
+  if (document.visibilityState === 'visible' && (phase.value === 'challenge' || phase.value === 'waiting' || phase.value === 'between')) {
+    pollForChallenge()
+  }
+}
+
 // BUG-5: Keyboard shortcuts for MC choices (A-D or 1-4)
 function handleKeyboard(e: KeyboardEvent) {
   if (phase.value !== 'challenge' || !currentChallenge.value?.choices?.length) return