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/locales/en/main.json

@@ -1845,8 +1845,21 @@
       "maxAmount": "(Max. $1,000 USD)",
       "buyNow": "Buy now",
       "seeDetails": "See details",
-      "topUp": "Top Up"
+      "topUp": "Top Up",
+      "addMoreCredits": "Add more credits",
+      "creditsDescription": "Credits are used to run workflows or partner nodes.",
+      "howManyCredits": "How many credits would you like to add?",
+      "videosEstimate": "~{count} videos*",
+      "templateNote": "*Generated with Wan Fun Control template",
+      "buy": "Buy",
+      "purchaseSuccess": "Purchase Successful",
+      "purchaseSuccessDetail": "Successfully purchased {credits} credits for {amount}",
+      "purchaseError": "Purchase Failed",
+      "purchaseErrorDetail": "Failed to purchase credits: {error}",
+      "unknownError": "An unknown error occurred"
     },
+    "creditsAvailable": "Credits available",
+    "refreshes": "Refreshes {date}",
     "eventType": "Event Type",
     "details": "Details",
     "time": "Time",