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)

backend/internal/service/account_stats_pricing.go

@@ -57,7 +57,8 @@ func tryModelFilePricing(billingService *BillingService, model string, tokens Us
 	cost := float64(tokens.InputTokens)*pricing.InputPricePerToken +
 		float64(tokens.OutputTokens)*pricing.OutputPricePerToken +
 		float64(tokens.CacheCreationTokens)*pricing.CacheCreationPricePerToken +
-		float64(tokens.CacheReadTokens)*pricing.CacheReadPricePerToken
+		float64(tokens.CacheReadTokens)*pricing.CacheReadPricePerToken +
+		float64(tokens.ImageOutputTokens)*pricing.ImageOutputPricePerToken
 	if cost <= 0 {
 		return nil
 	}
@@ -194,7 +195,7 @@ func calculateTokenStatsCost(pricing *ChannelModelPricing, tokens UsageTokens) *
 		float64(tokens.CacheCreationTokens)*deref(pricing.CacheWritePrice) +
 		float64(tokens.CacheReadTokens)*deref(pricing.CacheReadPrice) +
 		float64(tokens.ImageOutputTokens)*deref(pricing.ImageOutputPrice)
-	if cost == 0 {
+	if cost <= 0 {
 		return nil
 	}
 	return &cost