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/composables/node/useCreditsBadge.test.ts

@@ -17,6 +17,14 @@ vi.mock('@/stores/workspace/colorPaletteStore', () => ({
   })
 }))
 
+vi.mock('@/composables/useFeatureFlags', () => ({
+  useFeatureFlags: () => ({
+    flags: {
+      subscriptionTiersEnabled: false // Test legacy badge behavior
+    }
+  })
+}))
+
 const { updateSubgraphCredits } = usePriceBadge()
 
 const mockNode = new LGraphNode('mock node')