add shared comfy credit conversion helpers (#7061)

Introduces cents<->usd<->credit converters plus basic formatters and
adds test. Lays groundwork to start converting UI components into
displaying comfy credits.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7061-add-shared-comfy-credit-conversion-helpers-2bb6d73d3650810bb34fdf9bb3fc115b)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Claude <noreply@anthropic.com>

tests-ui/tests/base/credits/comfyCredits.test.ts

@@ -0,0 +1,46 @@
+import { describe, expect, test } from 'vitest'
+
+import {
+  CREDITS_PER_USD,
+  COMFY_CREDIT_RATE_CENTS,
+  centsToCredits,
+  creditsToCents,
+  creditsToUsd,
+  formatCredits,
+  formatCreditsFromCents,
+  formatCreditsFromUsd,
+  formatUsd,
+  formatUsdFromCents,
+  usdToCents,
+  usdToCredits
+} from '@/base/credits/comfyCredits'
+
+describe('comfyCredits helpers', () => {
+  test('exposes the fixed conversion rate', () => {
+    expect(CREDITS_PER_USD).toBe(211)
+    expect(COMFY_CREDIT_RATE_CENTS).toBeCloseTo(2.11) // credits per cent
+  })
+
+  test('converts between USD and cents', () => {
+    expect(usdToCents(1.23)).toBe(123)
+    expect(formatUsdFromCents({ cents: 123, locale: 'en-US' })).toBe('1.23')
+  })
+
+  test('converts cents to credits and back', () => {
+    expect(centsToCredits(100)).toBe(211) // 100 cents = 211 credits
+    expect(creditsToCents(211)).toBe(100) // 211 credits = 100 cents
+  })
+
+  test('converts USD to credits and back', () => {
+    expect(usdToCredits(1)).toBe(211) // 1 USD = 211 credits
+    expect(creditsToUsd(211)).toBe(1) // 211 credits = 1 USD
+  })
+
+  test('formats credits and USD values using en-US locale', () => {
+    const locale = 'en-US'
+    expect(formatCredits({ value: 1234.567, locale })).toBe('1,234.57')
+    expect(formatCreditsFromCents({ cents: 100, locale })).toBe('211.00')
+    expect(formatCreditsFromUsd({ usd: 1, locale })).toBe('211.00')
+    expect(formatUsd({ value: 4.2, locale })).toBe('4.20')
+  })
+})