Re-split library and preview when the editor is hidden; remember the editor's width

src/app/App.tsx

@@ -3,6 +3,7 @@ import { ConfirmDialog } from './components/ConfirmDialog';
 import { LivePreview } from './components/LivePreview';
 import { ModalShell } from './components/ModalShell';
 import { Onboarding } from './components/Onboarding';
+import { PaneSplitHandle } from './components/PaneSplitHandle';
 import { PaneToggleStrip } from './components/PaneToggleStrip';
 import { ResizeHandle } from './components/ResizeHandle';
 import { SnippetLibrary } from './components/SnippetLibrary';
@@ -143,10 +144,16 @@ export function App() {
                 <SnippetLibrary />
               </section>
             )}
-            {/* A resize handle only sits between two visible panes that flank the editor. */}
+            {/* A resize handle sits between any two adjacent visible panes. With the
+                editor present it flanks the editor (it absorbs the drag); with the
+                editor hidden, the library and preview become adjacent and share a
+                single split handle between them (spec §01A). */}
             {libraryVisible && editorVisible && (
               <ResizeHandle side="library" label="Resize snippet library" />
             )}
+            {libraryVisible && previewVisible && !editorVisible && (
+              <PaneSplitHandle label="Resize library and preview" />
+            )}
             {editorVisible && (
               <section id="pane-editor" className={styles.paneEditor} aria-label="Spec editor">
                 <SpecEditor />