feat: implement isSelectableGroup function to filter out unselectable groups

[masskrdjn]

• Resolves: [Bug]: Hide "unselectable" groups in account manager #57992

Summary

Hide the virtual guests app group from the account manager groups picker.

The guests app exposes a virtual guest_app group with the display name Guests. This group cannot be manually assigned from the accounts UI, and selecting it would not apply guest restrictions. This PR treats it like the existing internal groups that are not selectable in the picker, while keeping existing membership data untouched.

TODO

• Regenerate/update frontend dist if required by CI

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

Testing

• npx vitest --config vitest.local.config.ts run
  • 46 tests passed
• git diff --check
  • OK

Note: I could not regenerate the legacy frontend dist locally. The full legacy build ran out of memory first, then timed out after 20 minutes even with NODE_OPTIONS=--max-old-space-size=8192.

AI (if applicable)

• The content of this PR was partly generated using AI