fix(design): FINDING-011 follow-up — the create action is a real button

A 9.5px dim word in the rail header was technically an affordance and practically
invisible (user: 太不起眼). It is now a full-width 246x28 button directly under the
search row, same border and well as the field above it, ＋ 新零件 on the left and its
⌘N hint on the right, hover fill and brightening text.

It still carries no accent: DESIGN.md section 5 gives the composer the one lit action
per state. The Module `headerAction` prop added for the header version is removed
again rather than left as unused API.