fix: change quota notify threshold semantics to "remaining quota"

Threshold now represents remaining quota instead of usage amount:
- Fixed ($): threshold=400, limit=1000 → alert when remaining drops to $400
  (i.e., usage reaches $600)
- Percentage (%): threshold=30%, limit=1000 → alert when remaining drops
  to 30% (i.e., usage reaches $700)

Also:
- Rename 告警阈值 → 提醒阈值 in i18n
- Widen type dropdown to w-16 for proper $ / % display

frontend/src/i18n/locales/en.ts

@@ -2264,7 +2264,7 @@ export default {
       quotaLimitAmount: 'Total Limit',
       quotaLimitAmountHint: 'Cumulative spending limit. Does not auto-reset.',
       quotaNotify: {
-        alert: 'Alert Threshold',
+        alert: 'Alert',
         enabled: 'Enable Alert',
         threshold: 'Alert Amount',
         thresholdPlaceholder: 'Enter percentage',