fix: Restrict member management for non-admins/non-owners

[Dhanus3133]

What does this PR do?

This PR fixes incorrect behavior where members without the proper role could perform member management actions.

Changes made:

• Disabled Add user button for non-Owner/Admin users.
• Disabled Cancel invitation option in the member modal for non-Owner/Admin users.
• Disabled Remove member option in the member modal for non-Owner/Admin users.
• Disabled Edit roles option in the member modal for non-Owner/Admin users.
• Restricted access to the Edit Members page for non-Owner/Admin users.

The fix includes both UI restrictions and action-level checks. While hiding buttons in the UI prevents accidental misuse, enforcing role validation at the action level ensures security even if a user tries to bypass the UI by inspecting dev tools and manually enabling restricted options.

• Fixes [BUG] - Falsely shows success for member role update #1383

Visual Demo (For contributors especially)

Mandatory Tasks (DO NOT REMOVE)

• I have self-reviewed the code (A decent size PR without self-review might be rejected).
• I have updated the developer docs in /docs if this PR makes changes that would require a documentation change. If N/A, write N/A here and check the checkbox.
• I confirm automated tests are in place that prove my fix is effective or that my feature works.

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