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/views/admin/SettingsView.vue

@@ -2804,7 +2804,7 @@ import type {
   WebSearchProviderConfig,
   WebSearchTestResult,
 } from '@/api/admin/settings'
-import type { AdminGroup, Proxy } from '@/types'
+import type { AdminGroup, Proxy, NotifyEmailEntry } from '@/types'
 import type { ProviderInstance } from '@/types/payment'
 import AppLayout from '@/components/layout/AppLayout.vue'
 import Icon from '@/components/icons/Icon.vue'
@@ -3028,7 +3028,7 @@ const form = reactive<SettingsForm>({
   balance_low_notify_enabled: false,
   balance_low_notify_threshold: 0,
   account_quota_notify_enabled: false,
-  account_quota_notify_emails: [] as { email: string; disabled: boolean; verified: boolean }[]
+  account_quota_notify_emails: [] as NotifyEmailEntry[]
 })
 
 // Proxies for web search emulation ProxySelector