Three more visual reconciliations on the studio shell.

The vertical gutters that used to live on <main> moved onto the chrome itself, so the chrome carries its own margins and the main area is edge-to-edge. The header stopped floating: it is flat and static above the scroller now, so nothing floats over the content when you scroll. And the prompt rows above the composer picked up a real reusable component (the same look and behavior as the dock, plus a sticky push-away so a long prompt list does not shove the composer off the bottom).