fix: hide bounty actions from unauthorized users

[Thecesar85]

Summary

• only render the Edit Amount and Delete controls for org admins and mods
• keep the existing backend authorization checks intact
• remove the confusing UI path for viewers who cannot perform those actions

Closes #238

Validation

• reviewed AlgoraWeb.Org.BountiesLive to confirm current_user_role is already assigned by org navigation
• unable to run mix in this environment because Elixir tooling is not installed on this machine

[CLAassistant]

All committers have signed the CLA.