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/platform/cloud/subscription/components/SubscriptionPanel.vue

@@ -121,7 +121,7 @@
                       height="2rem"
                     />
                     <div v-else class="text-2xl font-bold">
-                      ${{ totalCredits }}
+                      {{ totalCredits }}
                     </div>
                   </div>
 
@@ -134,7 +134,7 @@
                         height="1rem"
                       />
                       <div v-else class="text-sm text-text-secondary font-bold">
-                        ${{ monthlyBonusCredits }}
+                        {{ monthlyBonusCredits }}
                       </div>
                       <div class="flex items-center gap-1">
                         <div class="text-sm text-text-secondary">
@@ -162,7 +162,7 @@
                         height="1rem"
                       />
                       <div v-else class="text-sm text-text-secondary font-bold">
-                        ${{ prepaidCredits }}
+                        {{ prepaidCredits }}
                       </div>
                       <div class="flex items-center gap-1">
                         <div class="text-sm text-text-secondary">