Register App Store bundle identifier (Mac App Store)

Part of: #9941

## Problem

The current bundle identifier `com.nextcloud.desktopclient` is already registered by another development team in App Store Connect. Apple does not allow two teams to own the same identifier, so the App Store submission will fail at the identifier step.

This cascades to every extension and the shared app group:

| Target | Current ID |
|---|---|
| Main app | `com.nextcloud.desktopclient` |
| FinderSyncExt | `com.nextcloud.desktopclient.FinderSyncExt` |
| FileProviderExt | `com.nextcloud.desktopclient.FileProviderExt` |
| FileProviderUIExt | `com.nextcloud.desktopclient.FileProviderUIExt` |
| App group | `NKUJUXUJ3B.com.nextcloud.desktopclient` |

## Required change

1. Agree on a new bundle ID (e.g. `com.nextcloud.desktop` or `io.nextcloud.desktopclient`) and register it in App Store Connect under Nextcloud GmbH's team (`NKUJUXUJ3B`).
2. Introduce a separate `APPLICATION_REV_DOMAIN_MAS` CMake variable (or reuse `APPLICATION_REV_DOMAIN` when `BUILD_FOR_APP_STORE=ON`) set to the new ID.
3. Update all four Info.plist templates, entitlements templates, and the app group identifier to use the new domain when building for the App Store.