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/game/fight/config.ts

@@ -2,6 +2,38 @@
 // Gameplay constants — all tunable magic numbers in one place
 // ═══════════════════════════════════════════════════════════════
 
+// --- Performance mode ---
+const PERF_KEY = 'bf_perf_mode'
+
+function detectLowEnd(): boolean {
+  if (typeof navigator === 'undefined') return false
+  const cores = navigator.hardwareConcurrency || 4
+  const mobile = navigator.maxTouchPoints > 0
+  return cores < 4 || mobile
+}
+
+function loadPerfPref(): boolean | null {
+  try {
+    const v = localStorage.getItem(PERF_KEY)
+    if (v === '1') return true
+    if (v === '0') return false
+  } catch {}
+  return null
+}
+
+let _perfMode = loadPerfPref() ?? detectLowEnd()
+
+export function isPerfMode(): boolean { return _perfMode }
+export function setPerfMode(on: boolean): void {
+  _perfMode = on
+  try { localStorage.setItem(PERF_KEY, on ? '1' : '0') } catch {}
+}
+
+/** Scale a particle count down in performance mode (40%) */
+export function perfParticles(count: number): number {
+  return _perfMode ? Math.max(1, Math.round(count * 0.4)) : count
+}
+
 // --- Choreography selection probabilities ---
 export const CREATOR_ULTIMATE_CHANCE = 0.5          // Creator ultimate chance (non-crit); 1.0 on crits
 export const CREATOR_THEMED_MOVE_RATIO = 0.6        // Creator uses creator-themed moves 60% of the time