perf: adaptive performance mode for mobile

Auto-detect low-end devices (< 4 cores or touch) and reduce visual
complexity: 60% fewer particles, 6 vs 16 speed lines, 15 vs 40 stars,
halved datacenter buildings/racks, skip LED blinking animations,
fewer GPU graveyard chips, reduced afterimages and exhaust particles.

Toggle in fight viewer (LO/HI button), persisted in localStorage.

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

frontend/src/components/FightViewer.vue

@@ -13,6 +13,7 @@ import {
   sfxRandomComedy, sfxRandomFail, sfxVineBoom, sfxEmotionalDamage,
 } from '../game/audio'
 import { isKokoroLoading, getKokoroProgress } from '../game/tts'
+import { isPerfMode, setPerfMode } from '../game/fight/config'
 
 interface Round {
   roundNumber: number
@@ -73,6 +74,7 @@ const hitTextY = ref(30)
 const glitching = ref(false)
 const soundOn = ref(true)
 const insanityMode = ref(false)
+const perfMode = ref(false)
 
 // Zap the winner
 const zapSent = ref(false)
@@ -155,6 +157,12 @@ function toggleInsanity() {
   }
 }
 
+perfMode.value = isPerfMode()
+function togglePerfMode() {
+  perfMode.value = !perfMode.value
+  setPerfMode(perfMode.value)
+}
+
 // Staggered log
 const logItems = ref<{ type: string; round: number; text: string; color: string }[]>([])
 
@@ -808,6 +816,16 @@ async function _doReplay() {
             <path d="M11 5L6 9H2v6h4l5 4V5z"/><line x1="23" y1="9" x2="17" y2="15"/><line x1="17" y1="9" x2="23" y2="15"/>
           </svg>
         </button>
+        <button
+          class="w-8 h-8 flex items-center justify-center border text-[9px] font-pixel tracking-wider transition-all"
+          :class="perfMode
+            ? 'border-neon-green/70 text-neon-green bg-neon-green/10'
+            : 'border-border/50 text-text-muted hover:text-neon-green hover:border-neon-green/50'"
+          :title="perfMode ? 'Performance mode ON (fewer particles)' : 'Performance mode OFF'"
+          @click="togglePerfMode"
+        >
+          {{ perfMode ? 'LO' : 'HI' }}
+        </button>
         <button
           v-if="isReplaying"
           class="h-8 px-2 flex items-center justify-center border text-[9px] font-pixel tracking-wider transition-all"