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/en.js

@@ -237,6 +237,11 @@ window.translations.en = {
     defaultCooldownDesc: "Fallback cooldown when API doesn't provide a reset time.",
     maxWaitThreshold: "Max Wait Before Error",
     maxWaitDesc: "If all accounts are rate-limited longer than this, error immediately instead of waiting.",
+    // Quota Protection
+    quotaProtection: "Quota Protection",
+    minimumQuotaLevel: "Minimum Quota Level",
+    minimumQuotaLevelDesc: "Switch accounts when quota drops below this level. Per-account overrides take priority.",
+    quotaDisabled: "Disabled",
     // Error Handling Tuning
     errorHandlingTuning: "Error Handling Tuning",
     rateLimitDedupWindow: "Rate Limit Dedup Window",