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/dialog/content/setting/LegacyCreditsPanel.vue

@@ -116,9 +116,9 @@ import { computed, ref, watch } from 'vue'
 import UserCredit from '@/components/common/UserCredit.vue'
 import UsageLogsTable from '@/components/dialog/content/setting/UsageLogsTable.vue'
 import Button from '@/components/ui/button/Button.vue'
+import { useBillingContext } from '@/composables/billing/useBillingContext'
 import { useFirebaseAuthActions } from '@/composables/auth/useFirebaseAuthActions'
 import { useExternalLink } from '@/composables/useExternalLink'
-import { useSubscription } from '@/platform/cloud/subscription/composables/useSubscription'
 import { useTelemetry } from '@/platform/telemetry'
 import { useDialogService } from '@/services/dialogService'
 import { useCommandStore } from '@/stores/commandStore'
@@ -138,7 +138,7 @@ const authStore = useFirebaseAuthStore()
 const authActions = useFirebaseAuthActions()
 const commandStore = useCommandStore()
 const telemetry = useTelemetry()
-const { isActiveSubscription } = useSubscription()
+const { isActiveSubscription } = useBillingContext()
 const loading = computed(() => authStore.loading)
 const balanceLoading = computed(() => authStore.isFetchingBalance)