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/pages/__tests__/__snapshots__/screens.snapshot.test.tsx.snap

@@ -5400,7 +5400,7 @@ exports[`screen snapshots > Run 1`] = `
         class="flex min-h-0 flex-1 gap-4"
       >
         <section
-          class="flex min-h-0 min-w-0 flex-1 flex-col "
+          class="flex min-h-0 min-w-0 flex-1 flex-col"
           data-testid="lane-detail"
         >
           <div