fix(storefront/a11y): swap focus-within for focus-visible on Button, restore focus rings on styled=false

[filiphsps]

What to build

The shared Button primitive uses focus-within:brightness-75 instead of focus-visible. This:

• Triggers on mouse-down (:focus-within matches on any descendant focus, including programmatic), producing the focus style for non-keyboard users.
• Provides no visible outline/ring — only a brightness change — which can be invisible on themed surfaces.

Additionally, several places pass styled={false} to Button (e.g. cart-button.tsx:19-31, "Clear cart" in cart-lines.tsx:34-45), which strips the existing focus styling without providing a replacement. Keyboard users get nothing.

Fix:

• Replace focus-within with focus-visible on the Button base.
• Add an explicit focus-visible:ring-2 focus-visible:ring-offset-2 (or token-equivalent) so the focused state is visible across light/dark surfaces.
• When styled={false}, still apply a minimum focus-visible ring at the wrapper.

Acceptance criteria

• No focus-within usage remains on Button (and any other interactive primitives where it was used as a focus proxy)
• Keyboard Tab through cart, header, and PDP produces a visible focus indicator on every interactive control
• styled={false} Buttons still surface focus
• Existing button tests updated; add focus-visible snapshot/assertion

Blocked by

None — can start immediately.

References

• apps/storefront/src/components/actionable/button.tsx:33
• apps/storefront/src/components/header/cart-button.tsx:19
• apps/storefront/src/components/cart/cart-lines.tsx:34