Add Achievements entry point to group sidebar Manage section

What does this MR do and why?

Adds an "Achievements" navigation item under the Manage section in the group sidebar, so users no longer need to know the URL to access group achievements.

This follows the same pattern as the existing Activity, Labels, and Members items — the link points to group_achievements_path and is gated behind the :read_achievement permission (which respects the :achievements feature flag).

Resolves #594737 (closed)

References

• Issue: #594737 (closed)
• Parent epic: &9429
• Approved navigation proposal: Proposal A

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the achievements feature flag in rails console:

   Feature.enable(:achievements)

2. Visit any group page, e.g. http://127.0.0.1:3000/groups/flightjs
3. In the left sidebar, expand the Manage section
4. Verify the Achievements item appears after Members
5. Click it and confirm it navigates to the group achievements page
6. Disable the feature flag and verify the item disappears:

   Feature.disable(:achievements)

MR acceptance checklist

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