(NII-Proposal) feat(files): add social media sharing feature flag and copy link option

[yacchin1205]

This PR is a simplified alternative to #841.

• Ticket: []
• Feature flag: disable_share_social_media

Purpose

Add ability to control social media sharing visibility via feature flag, and add a "Copy Link" option to the share menu.

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

Summary of Changes

• Add feature flag support (disable_share_social_media) to hide Twitter/Facebook share options when enabled
  • Changes for backend(osf.io): (NII-Proposal) Add disable_share_social_media feature flag osf.io#11561
• Add "Copy Link" option to share menu in file browser and file detail pages
• Email sharing remains available regardless of flag state

Screenshot(s)

disable_share_social_media flag disabled (default - social sharing enabled):

disable_share_social_media flag enabled (social sharing disabled):

Side Effects

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

QA Notes

• Test file context menu Share submenu on file list page
• Test Share dropdown on file detail page
• Verify "Copy Link" copies the file URL to clipboard
• With disable_share_social_media flag enabled: only Email and Copy Link should appear
• With flag disabled: Email, X, Facebook, and Copy Link should all appear