test(workspace): add split terminal view tests with corrected expectations

Added comprehensive tests for the split-terminal-view feature:
- Default 1-pane layout (no multi-pane UI)
- Switching to 2-pane and 4-pane layouts
- Persisting layout and lane selections to localStorage
- Fallback behavior when persisted lane IDs no longer exist

Fixed test expectation: when a persisted lane ID no longer exists and falls back
to unselected (null), both panes render as empty (pane-empty), not just one.

Updated Run snapshot to reflect the new layout toggle buttons.

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

@@ -5778,6 +5778,31 @@ exports[`screen snapshots > Run 1`] = `
     <div
       class="flex min-h-0 flex-col gap-2"
     >
+      <div
+        class="flex items-center gap-1.5"
+      >
+        <button
+          aria-pressed="true"
+          class="rounded border px-2 py-1 text-xs border-accent bg-accent/15 text-accent"
+          type="button"
+        >
+          1 pane
+        </button>
+        <button
+          aria-pressed="false"
+          class="rounded border px-2 py-1 text-xs border-border text-fg-secondary hover:border-border-light"
+          type="button"
+        >
+          2 pane
+        </button>
+        <button
+          aria-pressed="false"
+          class="rounded border px-2 py-1 text-xs border-border text-fg-secondary hover:border-border-light"
+          type="button"
+        >
+          4 pane
+        </button>
+      </div>
       <div
         class="flex min-h-0 flex-1 flex-col gap-5"
         data-testid="console-body"