Adds SureTextField, the form-control primitive following SureButton in the
mobile design-system sequence: a TextFormField wrapper that builds a complete,
brightness-aware InputDecoration from the active SureColors palette (filled
bg-container, borderSecondary hairline -> borderPrimary on focus, destructive
error border, textSubdued placeholder, radiusLg corners) with a DS-style label
rendered above the field and associated to it for screen readers (Material
labelText parity via ExcludeSemantics + Semantics(label:)).
Migrates the custom proxy-headers editor off raw TextFormField as proof.
Filter chips and a segmented control will follow as separate PRs.
Part of #2235.
