feat: Add a Dashboard link and route, and an optional redirect after login

Problem/Motivation

Depends on #3622694 (MR !1).

Sites that use gin_toolbar without core Toolbar never show the Dashboards toolbar tab, so nothing links a user to their dashboard.

Steps to reproduce

• Install a site with gin_toolbar and no core Toolbar module.
• Create a dashboard the user can view.
• Look for a way to reach it from the admin UI: there is none.

Proposed resolution

• Add a webdashboard.default route at /admin/webdashboard that redirects to the lowest-weight dashboard the user can view (403 when none), cacheable per permissions.
• Add a "Dashboard" admin menu link (parent system.admin) so Gin's toolbar shows it without core Toolbar.
• Add an optional "redirect after login" setting, off by default, with a settings form and a post_update hook.
• Document the feature in the README.

Remaining tasks

• ✅ File an issue about this project
• ✅ Addition/Change/Update/Fix to this project
• ✅ Testing to ensure no regression
• ✅ Automated unit/functional testing coverage
• ➖ Developer Documentation support on feature change/addition
• ➖ User Guide Documentation support on feature change/addition
• ➖ UX/UI designer responsibilities
• ➖ Accessibility and Readability
• ❌ Reviewed by a human
• ❌ Code review by maintainers
• ❌ Full testing and approval
• ❌ Credit contributors
• ❌ Review with the product owner
• ❌ Update Release Notes
• ❌ Release

User interface changes

• New "Dashboard" admin menu link.

API changes

• N/A

Data model changes

• New "redirect after login" setting on the Web Dashboard settings form.