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/app/blindbox/[code]/page.tsx

@@ -334,6 +334,7 @@ export default function BlindboxRoomPage() {
       <div className="flex w-full max-w-sm items-center gap-3">
         <button
           onClick={() => router.push("/blindbox")}
+          aria-label="返回"
           className="flex h-8 w-8 shrink-0 items-center justify-center rounded-full bg-surface ring-1 ring-border transition-colors active:bg-elevated"
         >
           <ArrowLeft size={16} className="text-muted" />
@@ -363,6 +364,7 @@ export default function BlindboxRoomPage() {
 
         <button
           onClick={() => setShowInvite(!showInvite)}
+          aria-label="邀请成员"
           className="flex h-8 w-8 shrink-0 items-center justify-center rounded-full bg-purple-600/15 text-purple-400 ring-1 ring-purple-500/20 transition-colors active:bg-purple-600/25"
         >
           <Share2 size={14} />
@@ -486,6 +488,7 @@ export default function BlindboxRoomPage() {
                   <button
                     onClick={handleSubmit}
                     disabled={!input.trim() || submitting}
+                    aria-label="提交想法"
                     className="flex h-12 w-12 items-center justify-center rounded-xl bg-purple-600 text-white transition-colors hover:bg-purple-500 disabled:opacity-30"
                   >
                     {submitting ? <Loader2 size={18} className="animate-spin" /> : <Send size={18} />}