feat: 移动端侧边栏体验 — 窄视口（<1024px）双工作台合并为单面板抽屉（可拖分隔条、高度与底部面板互通）、不提供底部面板按钮、文件/外链打开自动展开、新会话默认收起

src/client/sidebar.module.css

@@ -264,6 +264,38 @@
   display: flex;
 }
 
+/* The narrow-viewport workbench (MobileWorkbench): BOTH workbench trees
+   stacked in the one drawer — the right panel's tree flexes after the
+   fixed-height bottom tree, with a draggable divider (the .dividerCol
+   hit-strip + hairline) between them. */
+.mobileWorkbench {
+  flex: 1;
+  min-width: 0;
+  min-height: 0;
+  display: flex;
+  flex-direction: column;
+}
+
+.mobilePane {
+  position: relative;
+  display: flex;
+  min-width: 0;
+  min-height: 0;
+}
+
+/* The bottom tree of the merged layout: fixed height (state.bottomHeight),
+   flex: none so the top tree takes the remainder. */
+.mobilePaneBottom {
+  flex: none;
+}
+
+/* The divider of the merged layout sits between the two panes; .dividerCol
+   already supplies the 7px hit strip, row-resize cursor and hairline. */
+.mobileWorkbench > .dividerCol {
+  flex: none;
+  margin: 0;
+}
+
 .split {
   flex: 1;
   min-width: 0;
@@ -1864,3 +1896,28 @@
     animation: none;
   }
 }
+
+/* ── Narrow (mobile) viewport ──────────────────────────────────────────── */
+
+/* Pairs with NARROW_MAX_WIDTH = 1024 in src/client/breakpoints.ts: widths
+   strictly below 1024px (i.e. max-width: 1023px) switch the sidebar to the
+   mobile layout — one drawer holding both workbenches, one toggle button.
+   The structural switches (which panels render, the merged MobileWorkbench)
+   are JS-driven via useNarrowViewport; this block only adapts the pure
+   style details. */
+@media (max-width: 1023px) {
+  /* The toggle cluster holds a single button on narrow viewports (the
+     bottom panel button is not rendered), so the open panel's tab strip
+     reserves 40px instead of the desktop 72px — the tabs and + menu still
+     yield space to the one remaining control. */
+  .panel:not(.panelHidden) .tabBar {
+    padding-right: 40px;
+  }
+
+  /* Narrower tabs: a phone-width drawer fits more open tabs before the
+     strip scrolls. */
+  .tab {
+    min-width: 48px;
+    max-width: 128px;
+  }
+}