fix: AudioContext 复用 + 盲盒加载错误提示 + icon 按钮 aria-label

- #27: panic/blindbox/ShareCardModal/AuthModal 中 icon-only 按钮补 aria-label
- #28: playChime 缓存复用单个 AudioContext，避免超出浏览器 ~6 个上限
- #29: 已完成（上一批次 ApiError.name = "ApiError"）
- #30: blindbox lobby fetchRooms 失败时显示"加载失败/点击重试"
- #31: 已完成（theme.ts VALID_THEMES 校验）

src/components/AuthModal.tsx

@@ -188,6 +188,7 @@ export default function AuthModal({ open, onClose, onAuth, defaultTab = "login"
           <button
             type="button"
             onClick={() => setShowPassword(!showPassword)}
+            aria-label={showPassword ? "隐藏密码" : "显示密码"}
             className="absolute right-3 top-1/2 -translate-y-1/2 text-muted transition-colors active:text-secondary"
           >
             {showPassword ? <EyeOff size={16} /> : <Eye size={16} />}

src/components/ShareCardModal.tsx

@@ -130,6 +130,7 @@ export default function ShareCardModal({
             <div className="relative flex shrink-0 justify-center">
               <button
                 onClick={onClose}
+                aria-label="关闭"
                 className="absolute right-3 top-3 z-10 flex h-8 w-8 items-center justify-center rounded-full bg-black/40 text-white/70 transition-colors active:bg-black/60"
               >
                 <X size={16} />