Feat/workspaces 6 billing (#8508)

## Summary

Implements billing infrastructure for team workspaces, separate from
legacy personal billing.

## Changes

- **Billing abstraction**: New `useBillingContext` composable that
switches between legacy (personal) and workspace billing based on
context
- **Workspace subscription flows**: Pricing tables, plan transitions,
cancellation dialogs, and payment preview components for workspace
billing
- **Top-up credits**: Workspace-specific top-up dialog with polling for
payment confirmation
- **Workspace API**: Extended with billing endpoints (subscriptions,
invoices, payment methods, credits top-up)
- **Workspace switcher**: Now displays tier badges for each workspace
- **Subscribe polling**: Added polling mechanisms
(`useSubscribePolling`, `useTopupPolling`) for async payment flows

## Review Focus

- Billing flow correctness for workspace vs legacy contexts
- Polling timeout and error handling in payment flows

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8508-Feat-workspaces-6-billing-2f96d73d365081f69f65c1ddf369010d)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

src/components/toast/GlobalToast.vue

@@ -1,5 +1,13 @@
 <template>
   <Toast />
+  <Toast group="billing-operation" position="top-right">
+    <template #message="slotProps">
+      <div class="flex items-center gap-2">
+        <i class="pi pi-spin pi-spinner text-primary" />
+        <span>{{ slotProps.message.summary }}</span>
+      </div>
+    </template>
+  </Toast>
 </template>
 
 <script setup lang="ts">