Per-share toggle to disable activity notifications for internal shares

Is your feature request related to a problem? Please describe.

When sharing a folder with write access to an internal user, there is currently no way to prevent that recipient from receiving activity notifications (e.g. "a file has been changed") for that specific share.

The activity model is user × event type, configured in personal settings, not per share. This causes two problems:

• Disabling "a file or folder has been changed" turns notifications off for the recipient's entire activity, not just the shared folder.
• As the owner/admin I can force the recipient's setting via occ user:setting <user> activity notify_email_file_changed 0, but it is the recipient's personal preference — they can re-enable it, and there is no per-share lock.
• There is no per-folder / per-share granularity for internal shares.

Concrete use case: I share a personal working folder in write mode with a collaborator who must add and modify files, but who should not be subscribed to the activity stream of that folder.

Describe the solution you'd like

A toggle in the share's advanced settings (the sidebar panel for an internal user/group share) to enable or disable activity notifications for that specific share.

This is the natural level for the setting, since a notification is intrinsically tied to a share (folder + recipient). The "Advanced settings" section of an internal share already hosts options like expiration date, download/sync, and note to recipient — a notification toggle would fit right there.

Additional context

The closed issue #45449 implemented per-share notification for public link shares (shipped in Nextcloud 30, branch feat(activity): add share link upload notifications). During that work, internal shares were explicitly deferred ("agreement to skip internal completely for now"). This request asks to complete that work by bringing the same per-share control to internal shares — and specifically to support disabling notifications, not only enabling them.

This would also fit well with the ongoing sharing sidebar redesign (#50375, #48925) that separates internal and external shares.

Tested on Nextcloud 33.0.5 — no notification option is present in the advanced settings of an internal share.

Start by reviewing issue #45449 and the sharing sidebar redesign issues #50375 and #48925, then inspect how advanced settings are rendered for internal shares. Done means an internal share has a per-share notification toggle that can disable activity notifications without changing the recipient's global activity preference.