[NO-ISSUE] feat(webkit): switch contrast, concentric radius & theme-stable handle

[gab-az]

Summary

Design enhancements to the Switch component (inputs/switch), addressing a dark-mode contrast bug and aligning shape with the token system.

• Dark-mode contrast fix (checked state). The on-state handle previously used --bg-canvas and the lock icon --accent-contrast; in dark mode both resolved to black, so the lock icon (and the knob itself) had no contrast. The checked handle now uses --color-base-white and the lock icon --color-base-black — theme-independent base primitives, so the white knob + black lock stay fixed and legible in both light and dark.
• Checked track color moved to --accent (was the stale --success-contrast green).
• Shape / concentric radius. Track uses rounded-[var(--shape-elements)] (6px); handle uses rounded-[var(--radius)] (4px), concentric with the track (6px track − px-0.5 2px inset).
• Spec updated (.specs/switch.md): Tokens table + Theme gaps reflect the above; body checksum recomputed.

Notes

• No public API change — props, events, slots, and exports are unchanged (not breaking).
• The two theme-independent colors are intentional base primitives; a follow-up TODO is recorded in the spec's Theme gaps to promote them to semantic --fg-handle-on tokens.
• Verified: validate-spec-compliance, validate-tokens, ESLint, and vue-tsc type-check all pass. Full Storybook build not run locally.