feat: Add the Welcome and Site overview dashboard items

Problem/Motivation

The default dashboards look plain: a large chart, a raw table, and Site status rows without visible text in the core Default Admin theme.

Proposed resolution

• A Welcome item: the user picture (or initials), a greeting, member since, roles and quick actions.
• A Site overview item: stat cards for published and unpublished content, people, form submissions and site health.
• Two SDC components for them: welcome and stat_card.
• Use the full-link admin list markup for the Default Admin theme too, so the Site status rows show their text.
• Add content lists every content type when none are chosen; round charts keep a readable size.

Remaining tasks

• ✅ Implement
• ❌ Review

AI-Generated: Yes