Rename DuoChatHelper and fix disabled empty state card border

What does this MR do and why?

Addresses the two non-test follow-up items from #596538 (closed) (a follow-up from !228881 (merged)):

• Renames the DuoChatHelper module to PanelCommon. It is not a Rails helper, and "helper" is an overloaded term in Rails; the new name matches the sibling concern modules (PanelAutoExpand, LegacyCallout) and reflects that it holds logic shared across several panel components.
• Fixes the broken rounded-corner border on the "Steps to turn on GitLab Duo" card in DuoDisabledEmptyState. The gl-p-0 override on the gl-card root removed the default padding the card body's border-radius nests against, breaking the corners. Restoring a small padding fixes the border while keeping the layout intact.

The test-related follow-up items from #596538 (closed) are handled separately in !252382 (merged).

References

Related to #596538 (closed)

Screenshots or screen recordings

Before	After
broken rounded corner on the card border	card border renders correctly

How to set up and validate locally

1. As an Owner of a group/project with an Ultimate (or Duo) plan, turn GitLab Duo off for that container (Settings → GitLab Duo).
2. Ensure your user has a Duo seat assigned.
3. Open the group/project and expand the GitLab Duo panel.
4. Confirm the "Steps to turn on GitLab Duo" card renders with a clean, unbroken rounded border.
5. bundle exec rspec ee/spec/components/duo_chat_panel/
6. yarn jest ee/spec/frontend/ai/duo_agentic_chat/components/duo_disabled_empty_state_spec.js

MR acceptance checklist

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