[6.x] Give Switch controls an accessible name

[jaygeorge]

Description of the Problem

ui/Switch renders a role="switch" button whose only child is a decorative thumb. It had no accessible name unless a call site passed aria-label, so screen readers announced “switch, on” with no indication of what was being toggled.

Most CP call sites omitted a name — including the entry published-state toggle. axe-core reported critical button-name violations across several screens. WCAG 2.1 SC 4.1.2.

What this PR Does

• Adds a label prop on ui/Switch that sets aria-label
• Keeps fallthrough aria-label / aria-labelledby working for addons; warns in development when none is provided
• Passes real names at CP call sites (entry Published, sites, roles, conditions, validation, Bard/Replicator set enabled, Revealer, Scaffold, etc.)
• Uses aria-labelledby for Revealer when a visible input_label exists so the accessible name matches what’s on screen
• Documents the requirement in Storybook and adds unit coverage

Fixes #15391

How to Reproduce

1. Open any entry for editing
2. Inspect the Published toggle in the sidebar (before this PR)
3. Expected: an aria-label or aria-labelledby. Actual: <button role="switch" aria-checked="…"> with no name

Same on /cp/sites (site enabled) and /cp/roles/create (super user).