feat(settings): implement IDelegatedSettings for activity admin settings

[printminion-co]

Summary

Implements IDelegatedSettings for the activity admin settings page (OCA\Activity\Settings\Admin), instead of the plain ISettings interface. This allows an administrator to delegate management of the activity settings to a non-admin user or group via the admin delegation feature, without granting full admin rights.

• getName() returns the translated section label, Activity.
• getAuthorizedAppConfig() returns [] — no app config keys are exposed for direct delegated editing beyond the settings form itself.
• IL10N is already injected in this class, so the change is additive only.
• #[\Override] is set on both new methods, matching the rest of the class.

Companion to nextcloud/bruteforcesettings#1246, which makes the same change for the brute-force IP allowlist settings.

AI disclosure

This change was prepared with AI assistance (Claude Code): the cherry-pick and rebase mechanics onto current master, the #[\Override] consistency fix, and this PR description. The underlying code change and the DCO sign-off are from a human contributor.

Testing

• composer lint — clean
• composer cs:check — clean, 0 of 98 files need fixing
• composer psalm was not run locally: the pinned Psalm 7.0.0-beta19 requires PHP >= 8.3.16 and the available container has PHP 8.3.6. CI covers it here.
• No existing test touches this settings class.