Migrate Input to TS

[talissoncosta]

Part of #5746.

Input (Input.js) is the primitive behind ~94 files, but it's untyped and its styles are split across _input.scss and the junk-drawer _forms.scss. Migrating + tokenising it also lets us drop the dark-mode shim from #7745.

Scope

• Input.js → Input.tsx (typed props).
• Co-locate styles in Input.scss (merge the input rules from _input.scss + _forms.scss).
• Colours from semantic tokens (--color-border-default / -strong / -action) — drops the separate .dark block.
• Remove the feat: label variant key UI #7745 .dark .input-underline shim.

Out of scope

• Rest of _forms.scss (badge / notification / datepicker / segment dividers) — separate tickets.
• Field wrapper — done in Add unified Field component to standardise form labels, errors, and required indicators #7364.

Done when

• Input.tsx typed; no behavioural change for consumers (light + dark).
• Styles co-located + token-based; no hardcoded input colours.
• feat: label variant key UI #7745 shim removed; underline still correct in both themes.
• Input story covers the underline variant.