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/i18n/locales/en.ts

@@ -1889,7 +1889,8 @@ export default {
         searchAccountPlaceholder: 'Search accounts...',
         ruleAccountsHint: 'Leave empty to match all accounts',
         ruleModelPricing: 'Model Pricing',
-        noGroupsInChannel: 'No groups selected in platform tabs above'
+        noGroupsInChannel: 'No groups selected in platform tabs above',
+        unnamed: 'Unnamed'
       }
     },