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/composables/useFeatureFlags.ts

@@ -63,7 +63,7 @@ export function useFeatureFlags() {
         remoteConfig.value.subscription_tiers_enabled ??
         api.getServerFeature(
           ServerFeatureFlag.SUBSCRIPTION_TIERS_ENABLED,
-          false
+          true // Default to true (new design)
         )
       )
     }