(NII-Proposal) Add disable_share_social_media feature flag

[yacchin1205]

Ticket

• N/A (COS-NII meeting discussion)

Purpose

Add a new feature flag to control visibility of social media sharing options (Twitter/Facebook) in the Angular frontend.

This PR is based on discussions from COS-NII meetings.

Changes

• Add disable_share_social_media flag definition to osf/features.yaml
  • Changes for frontend(angular-osf): (NII-Proposal) feat(files): add social media sharing feature flag and copy link option angular-osf#857

Side Effects

None expected. The flag defaults to disabled (social sharing visible).

QE Notes

• This flag is consumed by the angular-osf frontend
• No backend behavior changes

CE Notes

• No migration required
• No server settings changes required
• Flag is auto-created on deployment via post_migrate signal

Documentation

• No external documentation updates required