fix(design): FINDING-011 — the rail has a visible create action

The create affordance existed only inside the empty state, so a workspace with
designs in it offered no way to start another one: the composer read 说一句话改这个
零件 and routed a typed sentence into the New Design surface, and ⌘N was real but
invisible. Reported by the user as 好像缺少 create new 的按钮.

The rail header now carries 新零件 (37x16, aria-label 新零件, title 新零件（⌘N）),
wired to the same create surface as ⌘N and the empty state. It is a word rather than
a glyph because the collapse control beside it is a +/− pair. The rail's own nested
card frame (border, radius, blur, shadow) is gone with it: the floating module plate
is the frame (section 22).