feat: init cloud calendar support (#14247)

#### PR Dependency Tree


* **PR #14247** 👈
  * **PR #14248**

This tree was auto-generated by
[Charcoal](https://github.com/danerwilliams/charcoal)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Google Calendar integration (disabled by default): link/unlink
accounts, OAuth flow, webhooks, real-time push, background sync,
workspace calendars with customizable items and date-range event
viewing.
* **GraphQL / Client**
* New queries & mutations for accounts, subscriptions, events,
providers, and workspace calendar management.
* **Localization**
* Added localized error message for calendar provider request failures.
* **Tests**
* Backend tests covering sync, webhook renewal, and error/error-recovery
scenarios.

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settings.</sub>
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/frontend/admin/src/config.json

@@ -255,6 +255,13 @@
       "desc": "Allowed version range of the app that allowed to access the server. Requires 'client/versionControl.enabled' to be true to take effect."
     }
   },
+  "calendar": {
+    "google": {
+      "type": "Object",
+      "desc": "Google Calendar integration config",
+      "link": "https://developers.google.com/calendar/api/guides/push"
+    }
+  },
   "captcha": {
     "enabled": {
       "type": "Boolean",