feat(workspace): session view and right panel sit on the page, no cards

The session section and the Files/Changes panel drop their cards; one
vertical divider separates them (none at full width), and the collapsed
rail keeps only that divider. The terminal goes back to the page colour
(--surface-0), and the panel's two sticky bars cover scrolled content in
the page colour instead of the card's.

Inside, the git row loses its bordered box for a bottom divider, and the
ended-session prompt box drops its own fill and shadow.

client/src/components/lanes/LaneGitHeader.tsx

@@ -276,10 +276,7 @@ export function LaneGitHeader({
     </>
   );
   return (
-    <div
-      data-testid="lane-git-header"
-      className="mb-2 rounded-md border border-border bg-surface-2 px-2 py-1"
-    >
+    <div data-testid="lane-git-header" className="mb-2 border-b border-border pb-1.5">
       <LaneGitBar
         laneId={laneId}
         session={session}