feat: per-account quota threshold protection (#212)

feat: per-account quota threshold protection

Resolves #135

- Adds configurable quota protection with three-tier threshold resolution (per-model → per-account → global)
- New global Minimum Quota Level slider in Settings
- Per-account threshold settings via Account Settings modal
- Draggable per-account threshold markers on model quota bars
- Backend: PATCH /api/accounts/:email endpoint, globalQuotaThreshold config
- i18n: quota protection keys for all 5 languages

public/js/translations/zh.js

@@ -237,6 +237,11 @@ window.translations.zh = {
     defaultCooldownDesc: "当 API 未提供重置时间时的备用冷却时间。",
     maxWaitThreshold: "最大等待阈值",
     maxWaitDesc: "如果所有账号的限流时间超过此阈值，立即返回错误而非等待。",
+    // 配额保护
+    quotaProtection: "配额保护",
+    minimumQuotaLevel: "最低配额水平",
+    minimumQuotaLevelDesc: "当配额低于此水平时切换账号。每个账号的单独设置优先。",
+    quotaDisabled: "已禁用",
     // 错误处理调优
     errorHandlingTuning: "错误处理调优",
     rateLimitDedupWindow: "限流去重窗口",