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>

src/base/credits/comfyCredits.ts

@@ -0,0 +1,125 @@
+const DEFAULT_NUMBER_FORMAT: Intl.NumberFormatOptions = {
+  minimumFractionDigits: 2,
+  maximumFractionDigits: 2
+}
+
+const formatNumber = ({
+  value,
+  locale,
+  options
+}: {
+  value: number
+  locale?: string
+  options?: Intl.NumberFormatOptions
+}): string => {
+  const merged: Intl.NumberFormatOptions = {
+    ...DEFAULT_NUMBER_FORMAT,
+    ...options
+  }
+
+  if (
+    typeof merged.maximumFractionDigits === 'number' &&
+    typeof merged.minimumFractionDigits === 'number' &&
+    merged.maximumFractionDigits < merged.minimumFractionDigits
+  ) {
+    merged.minimumFractionDigits = merged.maximumFractionDigits
+  }
+
+  return new Intl.NumberFormat(locale, merged).format(value)
+}
+
+export const CREDITS_PER_USD = 211
+export const COMFY_CREDIT_RATE_CENTS = CREDITS_PER_USD / 100 // credits per cent
+
+export const usdToCents = (usd: number): number => Math.round(usd * 100)
+
+export const centsToCredits = (cents: number): number =>
+  Math.round(cents * COMFY_CREDIT_RATE_CENTS)
+
+export const creditsToCents = (credits: number): number =>
+  Math.round(credits / COMFY_CREDIT_RATE_CENTS)
+
+export const usdToCredits = (usd: number): number =>
+  Math.round(usd * CREDITS_PER_USD)
+
+export const creditsToUsd = (credits: number): number =>
+  Math.round((credits / CREDITS_PER_USD) * 100) / 100
+
+export type FormatOptions = {
+  value: number
+  locale?: string
+  numberOptions?: Intl.NumberFormatOptions
+}
+
+export type FormatFromCentsOptions = {
+  cents: number
+  locale?: string
+  numberOptions?: Intl.NumberFormatOptions
+}
+
+export type FormatFromUsdOptions = {
+  usd: number
+  locale?: string
+  numberOptions?: Intl.NumberFormatOptions
+}
+
+export const formatCredits = ({
+  value,
+  locale,
+  numberOptions
+}: FormatOptions): string =>
+  formatNumber({ value, locale, options: numberOptions })
+
+export const formatCreditsFromCents = ({
+  cents,
+  locale,
+  numberOptions
+}: FormatFromCentsOptions): string =>
+  formatCredits({
+    value: centsToCredits(cents),
+    locale,
+    numberOptions
+  })
+
+export const formatCreditsFromUsd = ({
+  usd,
+  locale,
+  numberOptions
+}: FormatFromUsdOptions): string =>
+  formatCredits({
+    value: usdToCredits(usd),
+    locale,
+    numberOptions
+  })
+
+export const formatUsd = ({
+  value,
+  locale,
+  numberOptions
+}: FormatOptions): string =>
+  formatNumber({
+    value,
+    locale,
+    options: numberOptions
+  })
+
+export const formatUsdFromCents = ({
+  cents,
+  locale,
+  numberOptions
+}: FormatFromCentsOptions): string =>
+  formatUsd({
+    value: cents / 100,
+    locale,
+    numberOptions
+  })
+
+/**
+ * Clamps a USD value to the allowed range for credit purchases
+ * @param value - The USD amount to clamp
+ * @returns The clamped value between $1 and $1000, or 0 if NaN
+ */
+export const clampUsd = (value: number): number => {
+  if (Number.isNaN(value)) return 0
+  return Math.min(1000, Math.max(1, value))
+}