Add resizable panes with drag handles, min widths, and persistence

src/app/App.module.css

@@ -40,28 +40,24 @@
   min-height: 0;
 }
 
+/*
+ * Side panes (library, preview) carry an explicit width (set inline from the
+ * PanesStore) and don't grow or shrink — the drag handles change that width.
+ * The editor between them flexes to fill the remainder, so a drag leaves the
+ * opposite side pane untouched (spec §01A). Panes are separated by the
+ * ResizeHandle, so no inter-pane borders here.
+ */
 .pane {
-  flex: 1 1 0;
+  flex: 0 0 auto;
   min-width: 0;
   overflow: auto;
-  border-right: var(--border-width) solid var(--border);
   background: var(--bg);
 }
 
-/* Library is a fixed-ish sidebar; editor + preview share the rest. */
-.panes > .pane:first-child {
-  flex: 0 0 280px;
-}
-
 /* Editor pane: Monaco manages its own scroll/layout, so no padding. */
 .paneEditor {
   flex: 1 1 0;
   min-width: 0;
   overflow: hidden;
-  border-right: var(--border-width) solid var(--border);
   background: var(--bg);
 }
-
-.pane:last-child {
-  border-right: none;
-}