Move merge request actions to panel header

What does this MR do and why?

Consolidates the merge request title actions (Edit, Code, To-Do, notifications, and the more-actions dropdown) into a single Vue component that teleports them into the static panel header actions slot via PanelActionsPortal.

This aligns the merge request page with the panel layout already used by work items, and removes the now-duplicated To-Do and notification widgets from the tabs actions bar.

Highlights:

• New MrTitleActions component wraps the actions and renders them through PanelActionsPortal.
• The Edit button is migrated from HAML to Vue.
• To-Do, notification, Code, and more-actions controls use tertiary, small buttons for consistency in the panel header.
• The more-actions dropdown now uses the horizontal ellipsis icon.

References

• gitlab-org/upstream-studios/design-strategy/craft-quotient#41

Follows the panel header actions pattern documented in doc/development/fe_guide/page_layouts.md.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Open any merge request (for example http://gdk.test:3000/<group>/<project>/-/merge_requests/1).
2. Confirm the Edit, Code, To-Do, notification, and more-actions controls now appear in the panel header actions area.
3. Confirm the To-Do/notification widgets no longer appear in the tabs actions bar.

MR acceptance checklist

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