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/SessionReplay.tsx

@@ -86,7 +86,7 @@ export function SessionReplay({
       <div className="min-h-0 flex-1 overflow-hidden">
         <ConversationView sessionId={sessionId} />
       </div>
-      <div className="rounded-2xl border border-border bg-surface-1 p-2 shadow-sm transition-colors focus-within:border-accent/50">
+      <div className="rounded-2xl border border-border p-2 transition-colors focus-within:border-accent/50">
         <PromptEditor
           bare
           images={images}