Add error handling for GraphQL mutations across codebase

[coderabbitai]

Context

During review of PR #300 (#300), it was identified that GraphQL mutation calls currently lack error handling throughout the codebase. Network errors or server-side failures are silently swallowed with no user-visible feedback.

Scope

This issue tracks adding proper error handling for all GraphQL mutation calls across the application to ensure:

• Mutation failures are caught and handled
• Users receive appropriate feedback when operations fail
• Error states are surfaced in the UI (e.g., via toasts, alerts, or error messages)

References

• Original discussion: Added user access management page #300 (comment)
• Requested by: @danielemery