fix(usage): prefer explicit video billing mode over image_count

Reuse shared getDisplayBillingMode so user usage rows with billing_mode=video
are not mislabeled as image when image_count is non-zero.

frontend/src/utils/__tests__/billingMode.spec.ts

@@ -0,0 +1,27 @@
+import { describe, expect, it } from 'vitest'
+import {
+  BILLING_MODE_IMAGE,
+  BILLING_MODE_TOKEN,
+  BILLING_MODE_VIDEO,
+  getDisplayBillingMode,
+  isImageUsage
+} from '../billingMode'
+
+describe('billingMode helpers', () => {
+  it('prefers explicit video mode over image_count', () => {
+    expect(
+      getDisplayBillingMode({ image_count: 1, billing_mode: BILLING_MODE_VIDEO })
+    ).toBe(BILLING_MODE_VIDEO)
+    expect(isImageUsage({ image_count: 1, billing_mode: BILLING_MODE_VIDEO })).toBe(false)
+  })
+
+  it('infers image when image_count set and mode missing', () => {
+    expect(getDisplayBillingMode({ image_count: 2, billing_mode: null })).toBe(BILLING_MODE_IMAGE)
+  })
+
+  it('keeps token mode even with image_count', () => {
+    expect(
+      getDisplayBillingMode({ image_count: 1, billing_mode: BILLING_MODE_TOKEN })
+    ).toBe(BILLING_MODE_TOKEN)
+  })
+})

frontend/src/utils/billingMode.ts

@@ -32,6 +32,10 @@ export function isImageUsage(row: Pick<ImageBillingRow, 'image_count' | 'billing
 }
 
 export function getDisplayBillingMode(row: Pick<ImageBillingRow, 'billing_mode' | 'image_count'> | null | undefined): string | null | undefined {
+  // Explicit video/token modes always win over image_count heuristics.
+  if (row?.billing_mode === BILLING_MODE_VIDEO || row?.billing_mode === BILLING_MODE_TOKEN) {
+    return row.billing_mode
+  }
   if ((row?.image_count ?? 0) > 0 && !row?.billing_mode) {
     return BILLING_MODE_IMAGE
   }