fix(ENG-9810): wire up provider subscription notification settings

[Vlad0n20]

• Ticket: [ENG-9810]
• Feature flag: fix

Purpose

Summary of Changes

Moderators of Collections, Registries, and Preprints need to control
how they receive notifications about moderation activity (new pending
submissions, withdrawal requests) — instant, daily, or none.
Previously, the Settings/Notifications tabs in the moderation pages
were empty placeholders that just pointed users away to their global
user settings. That was insufficient because these are provider-level
subscriptions (scoped to a specific provider), not global ones.

The changes wire up the real OSF API
(/v2/providers/{type}/{id}/subscriptions/) so each moderation portal's
notification tab now:

1. Loads the actual per-provider subscription preferences for the
   logged-in moderator
2. Lets them change the frequency via dropdowns, saved immediately via
   PATCH

Screenshot(s)

Side Effects

QA Notes