feat: watch another fight, rename nav — FIGHT! + WATCH, fix stale SW assets

- Add "WATCH ANOTHER FIGHT" button on post-fight overlay (loads random fight)
- Rename "JOIN A BOUT" → "FIGHT!" in nav, join page, register page
- Rename "ARENA" → "WATCH" in nav
- Server: return 404 for missing asset files instead of index.html (fixes "Unexpected token '<'")
- PWA: cleanupOutdatedCaches, skipWaiting, clientsClaim for instant SW updates
- Serve PWA files (sw.js, workbox, icons) with correct cache headers

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

frontend/src/pages/FightPage.vue

@@ -636,6 +636,29 @@ function onReplayDone() {
   }
 }
 
+const isLoadingNext = ref(false)
+async function watchAnotherFight() {
+  if (isLoadingNext.value) return
+  isLoadingNext.value = true
+  try {
+    const res = await fetch('/api/fights')
+    if (res.ok) {
+      const fights = await res.json()
+      const finished = fights.filter((f: any) => f.status === 'finished' && f.id !== fightId.value)
+      if (finished.length > 0) {
+        const pick = finished[Math.floor(Math.random() * finished.length)]
+        router.push(`/arena/${pick.id}`)
+        return
+      }
+    }
+    router.push('/arena')
+  } catch {
+    router.push('/arena')
+  } finally {
+    isLoadingNext.value = false
+  }
+}
+
 function startAutoBattle() {
   if (!myBotId.value || isHumanFight.value) return
   autoBattle.value = true
@@ -934,6 +957,17 @@ function stopAutoBattle() {
                 </button>
               </template>
 
+              <button
+                :disabled="isLoadingNext"
+                class="w-full py-3 bg-neon-purple/10 border-2 border-neon-purple/50 text-neon-purple
+                       font-display font-black text-sm tracking-widest rounded-lg
+                       hover:bg-neon-purple/20 hover:border-neon-purple transition-all
+                       disabled:opacity-50 disabled:cursor-wait flex items-center justify-center gap-2"
+                @click="watchAnotherFight"
+              >
+                <span v-if="isLoadingNext" class="w-4 h-4 border-2 border-neon-purple/30 border-t-neon-purple rounded-full animate-spin" />
+                {{ isLoadingNext ? 'LOADING...' : 'WATCH ANOTHER FIGHT' }}
+              </button>
               <button
                 class="w-full py-2 border border-white/10 text-text-muted
                        font-display text-xs tracking-widest rounded-lg