feat: #3629449 Give the welcome dashboard the Varbase dashboard layout

Issue: https://www.drupal.org/i/3629449

Change: the welcome dashboard is laid out like varbase_dashboards on Varbase 10.1.x. Configuration only.

• A welcome card on top of the left column: the user's picture (128×128 WebP avatar image style) with "Welcome," and the name beside it, from a small view of the current user.
• Add content (the Create menu) and Top tasks below it.
• My drafts, Recent content, Recent pages and accessibility alerts on the right, with Scheduled content underneath.
• No Drupal Events or Announcements block.
• Applies core's user_picture recipe, which the welcome card needs.

Needs Varbase Content Base !38 for My drafts and Scheduled to show for editors.

Tested on fresh Drupal CMS 2.2.3 installs of Varbase Starter (and earlier Horizon Aid) with the Varbase testing users:

• Editor, Content admin, Site admin: welcome card, Add content, Top tasks, My drafts, Recent content, Recent pages, Scheduled. Site admin also gets "Invite users to collaborate" in Top tasks.
• SEO admin: the same, without Recent pages.
• Super admin: everything, including all Top tasks links and Pages with accessibility alerts.
• Normal user: access denied.

Per-role screenshots are on the issue: https://www.drupal.org/project/varbase_admin_base/issues/3629449

Related: Varbase Admin Base !35 (merged), Varbase Content Base !38.

AI-Generated: Yes

Checkpoints:

• File an issue
• Addition/Change/Update/Fix
• Testing to ensure no regression
• Automated unit testing coverage
• Automated functional testing coverage
• UX/UI designer responsibilities
• Readability
• Accessibility
• Performance
• Security
• Developer Documentation
• User Guide Documentation
• Reviewed by human
• Code review by maintainers
• Full testing and approval
• Credit contributors
• Review with the product owner
• Release notes snippet
• Release