feat: #3622701 Add a Dashboard link and route, and an optional redirect after login

Adds an admin/webdashboard route and a "Dashboard" admin menu link, so sites using Gin's toolbar without core Toolbar can still reach their dashboard.

• New webdashboard.default route, redirecting to the lowest-weight dashboard the current user can view (403 when none), cacheable per permissions.
• New "Dashboard" admin menu link under system.admin.
• Optional "redirect after login" setting, off by default, with a settings-form addition and a post_update hook.
• README section documenting the feature.

Issue: https://www.drupal.org/project/webdashboard/issues/3622701

Stacked on !1 (merged) (#3622694): the diff shows its commit until !1 (merged) is merged, then this branch is rebased onto 12.0.x.

AI-Generated: Yes

Checkpoints

• 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