fix: address audit findings across websearch, notify, and channel pricing

Backend fixes:
- Fix balance notify ignoring percentage threshold type (was treating
  percentage value as fixed USD amount)
- Remove dead code parseJSONStringArray
- Add ImageOutputTokens to tryModelFilePricing calculation
- Unify zero-value check: cost == 0 → cost <= 0 in calculateTokenStatsCost
- Use MarshalNotifyEmails instead of json.Marshal for consistency
- Rename quotaDim.oldUsed → currentUsed for clarity
- Extract HTML email templates to const variables (function ≤30 lines)

Test fixes:
- Rewrite account_websearch_test.go for GetWebSearchEmulationMode tri-state
- Add 6 tryModelFilePricing test cases

Frontend fixes:
- Replace hardcoded '未命名' with i18n key
- Extract getBillingModeLabel/getBillingModeBadgeClass to shared utils
- Replace inline type with imported NotifyEmailEntry
- Pass platform to AccountStats pricing rules via inferRulePlatform()
- Add billing mode constants (BILLING_MODE_TOKEN/PER_REQUEST/IMAGE)

frontend/src/utils/billingMode.ts

@@ -0,0 +1,19 @@
+export const BILLING_MODE_TOKEN = 'token'
+export const BILLING_MODE_PER_REQUEST = 'per_request'
+export const BILLING_MODE_IMAGE = 'image'
+
+export function getBillingModeLabel(mode: string | null | undefined, t: (key: string) => string): string {
+  switch (mode) {
+    case BILLING_MODE_PER_REQUEST: return t('admin.usage.billingModePerRequest')
+    case BILLING_MODE_IMAGE: return t('admin.usage.billingModeImage')
+    default: return t('admin.usage.billingModeToken')
+  }
+}
+
+export function getBillingModeBadgeClass(mode: string | null | undefined): string {
+  switch (mode) {
+    case BILLING_MODE_PER_REQUEST: return 'bg-purple-100 text-purple-700 dark:bg-purple-900/30 dark:text-purple-300'
+    case BILLING_MODE_IMAGE: return 'bg-pink-100 text-pink-700 dark:bg-pink-900/30 dark:text-pink-300'
+    default: return 'bg-blue-100 text-blue-700 dark:bg-blue-900/30 dark:text-blue-300'
+  }
+}