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/breakpoints.ts

@@ -0,0 +1,50 @@
+/**
+ * Narrow-viewport ("mobile") breakpoint for the sidebar. Width-based, shared
+ * by the layout logic (JS) and the style gates (CSS). The CSS side pairs
+ * with this file via `@media (max-width: 1023px)` rules (sidebar.module.css)
+ * — 1023px ≡ widths below NARROW_MAX_WIDTH, documented at both ends.
+ *
+ * The value matches the DSH app shell's own narrow breakpoint
+ * (SIDEBAR_AUTO_COLLAPSE = 1024, ui-layout/columns.ts), so the sidebar's
+ * mobile layout switches at exactly the width where the host app collapses
+ * its own sidebar to the rail.
+ */
+import { useEffect, useState } from 'react'
+
+/** Viewport widths strictly below this are "mobile" (paired CSS: max-width: 1023px). */
+export const NARROW_MAX_WIDTH = 1024
+
+/** Whether a viewport width is narrow (mobile). */
+export function isNarrowWidth(width: number): boolean {
+  return width < NARROW_MAX_WIDTH
+}
+
+/**
+ * Live narrow-viewport flag for components. Reads `window.innerWidth` and
+ * re-measures on resize (rAF-throttled, the repo's existing drag pattern).
+ * Deliberately avoids `matchMedia` (jsdom does not implement it) — the
+ * resize listener is equally exact for a breakpoint that never changes
+ * while the page is open.
+ */
+export function useNarrowViewport(): boolean {
+  const [narrow, setNarrow] = useState(
+    () => typeof window !== 'undefined' && isNarrowWidth(window.innerWidth),
+  )
+  useEffect(() => {
+    if (typeof window === 'undefined') return
+    let frame: number | null = null
+    const measure = (): void => {
+      frame = null
+      setNarrow(isNarrowWidth(window.innerWidth))
+    }
+    const onResize = (): void => {
+      if (frame === null) frame = requestAnimationFrame(measure)
+    }
+    window.addEventListener('resize', onResize)
+    return () => {
+      window.removeEventListener('resize', onResize)
+      if (frame !== null) cancelAnimationFrame(frame)
+    }
+  }, [])
+  return narrow
+}