Fix Switch keyboard activation, focus ring, and thumb geometry

[burieberry]

Fixes the boxel-ui Switch per CS-12328:

• Single change binding replaces the click + keypress pair, which double-fired on Space and made the switch keyboard-inoperable; Enter (inert on checkboxes) gets its own keydown path
• The Switch owns a :focus-visible ring on the label; the UA halo on the inner input is suppressed
• Thumb travel is translateX(width − height) instead of margin-left: 49% — flush right at any size/border/padding, animated via transform
• onChange passes the new state (() => void callers stay assignable); aria-checked renders as explicit 'true'/'false' instead of vanishing when false
• Adds a shared --boxel-border-radius-pill token; the track uses it
• Template-only component; SwitchSiganture typo fixed

Verified through the homepage mode toggle: Space, Enter, and click each toggle exactly once; ring draws on the control; thumb lands flush at both ends. The boxel-home consumer workarounds are removed in a follow-up there, which should land after this deploys.

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