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

src/app/infrastructure/ux-prefs.ts

@@ -28,6 +28,8 @@ const KEY = 'astrolabe:ux-prefs';
 export interface PanelLayout {
   libraryWidth?: number;
   previewWidth?: number;
+  /** The editor's remembered width, so it re-shows at it when revealed (§01A). */
+  editorWidth?: number;
 }
 
 /** Per-pane visibility. Optional — a missing field falls back to "shown". */
@@ -91,6 +93,7 @@ export function loadPanelLayout(): PanelLayout {
   return {
     libraryWidth: posNumber(stored.libraryWidth),
     previewWidth: posNumber(stored.previewWidth),
+    editorWidth: posNumber(stored.editorWidth),
   };
 }