i18n: replace hardcoded English toast strings in SubscriptionRequiredDialogContentWorkspace.vue

## Summary

`src/platform/workspace/components/SubscriptionRequiredDialogContentWorkspace.vue` has 9 hardcoded English strings in toast messages (lines 170, 180, 195, 252, 255, 306, 309, 330, 333). All user-facing strings must use `t()` per AGENTS.md.

Examples:
- `'Unable to subscribe'`
- `'This plan is not available'`
- `'Error'`
- `'Failed to subscribe'`
- `'Failed to update subscription'`
- `'Failed to resubscribe'`

The personal variant (`SubscriptionRequiredDialogContent.vue`) avoids this because it delegates subscribing to child components, but the workspace variant inlines the subscribe flow and skips i18n for error toasts.

## Fix

1. Add the missing keys to `src/locales/en/main.json`
2. Replace all hardcoded strings with `t('subscription.')` calls

## References

- PR: https://github.com/Comfy-Org/ComfyUI_frontend/pull/9901
- Review: https://github.com/Comfy-Org/ComfyUI_frontend/pull/9901#pullrequestreview-3963889519

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-10455-i18n-replace-hardcoded-English-toast-strings-in-SubscriptionRequiredDialogContentWor-32d6d73d36508111a1d0fdfb7786bf3d) by [Unito](https://www.unito.io)

Read AGENTS.md and inspect src/platform/workspace/components/SubscriptionRequiredDialogContentWorkspace.vue at the listed lines, then compare the existing subscription entries in src/locales/en/main.json. Done means all nine user-facing toast strings use the specified translation pattern and the corresponding locale keys are present; PR 9901 provides related review context.

Written by the indexing model from the issue text.