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/LaneChangesPanel.tsx

@@ -984,7 +984,7 @@ export function LaneChangesPanel({
     <div data-testid="lane-changes-browse">
       {/* Pinned: the finder is what you reach for after scrolling a
                 long tree, not only from its top. */}
-      <div className="sticky top-0 z-10 flex items-center gap-1.5 bg-surface-3 pb-2">
+      <div className="sticky top-0 z-10 flex items-center gap-1.5 bg-surface-0 pb-2">
         <input
           value={query}
           onChange={(e) => setQuery(e.target.value)}
@@ -1320,7 +1320,7 @@ export function LaneChangesPanel({
               {(stagedCount > 0 || toStage.length > 0) && (
                 <div
                   data-testid="lane-changes-actions"
-                  className="sticky top-0 z-10 mb-1.5 flex items-center gap-1.5 border-b border-border-light bg-surface-3 pb-2"
+                  className="sticky top-0 z-10 mb-1.5 flex items-center gap-1.5 border-b border-border-light bg-surface-0 pb-2"
                 >
                   {/* ONE row: message, commit, then the two bulk index moves. */}
                   {stagedCount > 0 && (