fix(notify): address review findings - accountCost formula, dedup, refactor

- Fix accountCost calculation in finalizePostUsageBilling to match
  postUsageBilling (always multiply by AccountRateMultiplier)
- Use strings.EqualFold for email dedup in collectBalanceNotifyRecipients
- Extract CheckAccountQuotaAfterIncrement into smaller functions:
  buildQuotaDims + asyncSendQuotaAlert (< 30 lines each)
- Add "not splittable" comments for HTML template functions
- Extract QuotaNotifyToggle.vue sub-component to reduce
  QuotaLimitCard.vue from 404 to 339 lines

frontend/src/components/account/QuotaNotifyToggle.vue

@@ -0,0 +1,47 @@
+<script setup lang="ts">
+import { useI18n } from 'vue-i18n'
+
+const { t } = useI18n()
+
+defineProps<{
+  enabled: boolean | null
+  threshold: number | null
+}>()
+
+const emit = defineEmits<{
+  'update:enabled': [value: boolean | null]
+  'update:threshold': [value: number | null]
+}>()
+</script>
+
+<template>
+  <div class="ml-4 mt-2 flex items-center gap-3">
+    <label class="text-sm text-gray-500 whitespace-nowrap">{{ t('admin.accounts.quotaNotify.alert') }}</label>
+    <button
+      type="button"
+      @click="emit('update:enabled', !enabled)"
+      :class="[
+        'relative inline-flex h-5 w-9 flex-shrink-0 cursor-pointer rounded-full border-2 border-transparent transition-colors duration-200 ease-in-out focus:outline-none',
+        enabled ? 'bg-primary-600' : 'bg-gray-200 dark:bg-dark-600'
+      ]"
+    >
+      <span
+        :class="[
+          'pointer-events-none inline-block h-4 w-4 transform rounded-full bg-white shadow ring-0 transition duration-200 ease-in-out',
+          enabled ? 'translate-x-4' : 'translate-x-0'
+        ]"
+      />
+    </button>
+    <div v-if="enabled" class="relative flex-1">
+      <span class="absolute left-2 top-1/2 -translate-y-1/2 text-gray-400 text-sm">$</span>
+      <input
+        :value="threshold"
+        @input="emit('update:threshold', parseFloat(($event.target as HTMLInputElement).value) || null)"
+        type="number"
+        min="0"
+        step="0.01"
+        class="input pl-6 py-1 text-sm"
+      />
+    </div>
+  </div>
+</template>