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

tests/breakpoints.spec.ts

@@ -0,0 +1,23 @@
+/**
+ * Narrow-viewport breakpoint tests: the boundary the mobile sidebar layout
+ * keys off (paired with the CSS @media (max-width: 1023px) gates in
+ * sidebar.module.css — 1023px ≡ widths strictly below NARROW_MAX_WIDTH).
+ */
+import { describe, expect, it } from 'vitest'
+import { isNarrowWidth, NARROW_MAX_WIDTH } from '../src/client/breakpoints.ts'
+
+describe('narrow-viewport breakpoint', () => {
+  it('treats widths below NARROW_MAX_WIDTH as narrow (mobile)', () => {
+    expect(NARROW_MAX_WIDTH).toBe(1024)
+    expect(isNarrowWidth(320)).toBe(true)
+    expect(isNarrowWidth(390)).toBe(true)
+    expect(isNarrowWidth(768)).toBe(true)
+    expect(isNarrowWidth(1023)).toBe(true)
+  })
+
+  it('treats NARROW_MAX_WIDTH and above as wide (desktop)', () => {
+    expect(isNarrowWidth(1024)).toBe(false)
+    expect(isNarrowWidth(1280)).toBe(false)
+    expect(isNarrowWidth(1920)).toBe(false)
+  })
+})