ui(members): adopt PageHeader/PageBody and explain why no group can be added

[jamofer]

Summary

Two issues on the project members page, both visible the moment an admin or owner lands on it.

• Layout drift. MembersPage rendered its own <Breadcrumbs> + custom pl-14 pr-4 py-4 md:p-8 wrapper, so it didn't pick up the sticky header / centered body pattern that every other project page uses (Bugs, Executions, Plans, Versions, Coverage, Custom Fields). Refactored to the shared PageHeader + PageBody primitives, kept the breadcrumb trail, and centered the content with max-w-4xl. While here, the inline "add member" form gets flex-wrap so the three controls reflow on narrow viewports instead of overflowing.
• Silent group-access dead end. ProjectGroupsSection only rendered the assign form when there was at least one unassigned candidate group. If no groups existed yet, or every existing group was already on the project, the form vanished with no explanation and the admin had no way to know why. Added a small hint paragraph below the assignment list:
  • When no groups exist at all → "No groups exist yet. Create one in the Users admin (Groups tab) and it will appear here."
  • When all groups are already assigned → "Every existing group is already assigned to this project."
• New i18n keys groups.noCandidatesEmpty / groups.noCandidatesAllAssigned translated across EN/ES/CA/GL/EU.
• ProjectGroupsSection.test.jsx gets two new cases that mock the empty/exhausted candidate states and assert the right hint renders.

Frontend suite: 377 passing.