feat(organizations): use mantine in member actions dropdown TASK -1382

[jamesrkiger]

🗒️ Checklist

1. run linter locally
2. update all related docs (API, README, inline, etc.), if any
3. draft PR with a title <type>(<scope>)<!>: <title> TASK-1234
4. tag PR: at least frontend or backend unless it's global
5. fill in the template below and delete template comments
6. review thyself: read the diff and repro the preview as written
7. open PR & confirm that CI passes
8. request reviewers, if needed
9. delete this section before merging

📣 Summary

Replaces older custom components in the MemberActionsDropdown with Mantine menu.

👀 Preview steps

1. As an admin or Owner of an MMO with a few members, navigate to the organization members page.
2. Compare the actions dropdown between this branch and main. (note: We stuck with Mantine's default padding for the menu.)
3. On PR branch, remove a member to confirm menu actually works.

[notion-workspace]

Replace underlying component in MemberActionsDropdown (Mantine PoC)