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/platform/cloud/subscription/components/SubscriptionPanel.test.ts

@@ -17,9 +17,9 @@ const mockSubscriptionData = {
 }
 
 const mockCreditsData = {
-  totalCredits: '10.00',
-  monthlyBonusCredits: '5.00',
-  prepaidCredits: '5.00',
+  totalCredits: '10.00 Credits',
+  monthlyBonusCredits: '5.00 Credits',
+  prepaidCredits: '5.00 Credits',
   isLoadingBalance: false
 }
 
@@ -154,8 +154,8 @@ describe('SubscriptionPanel', () => {
   describe('credit display functionality', () => {
     it('displays dynamic credit values correctly', () => {
       const wrapper = createWrapper()
-      expect(wrapper.text()).toContain('$10.00') // totalCredits
-      expect(wrapper.text()).toContain('$5.00') // both monthlyBonus and prepaid
+      expect(wrapper.text()).toContain('10.00 Credits')
+      expect(wrapper.text()).toContain('5.00 Credits')
     })
 
     it('shows loading skeleton when fetching balance', () => {