polish(web): 侧边栏滚动细节

1. 会话列表底部留白 12→48px，滚动到底时最后一项不再被底部渐变遮住。
2. 侧边栏滚动条默认隐藏，仅在指针悬停侧边栏区域时显示（通过主题滚动条变量绑透明实现，Firefox/WebKit 两条渲染路径同时生效）。

packages/client/ui-sidebar/src/client/SidebarRoot.module.css

@@ -15,6 +15,16 @@
   background: var(--dsw-specific-sidebar-fill);
   color: var(--dsw-alias-label-primary);
   font-size: 14px;
+  /* Scrollbar hides until the pointer is over the sidebar: rebind the
+     ui-theme scrollbar indirection (styles/scrollbar.css) to transparent;
+     the session list's scrollbar-gutter keeps layout stable either way. */
+  --dsh-scrollbar-thumb: transparent;
+  --dsh-scrollbar-thumb-hover: transparent;
+}
+
+.root:hover {
+  --dsh-scrollbar-thumb: var(--dsw-alias-scrollbar-bg-l1);
+  --dsh-scrollbar-thumb-hover: var(--dsw-alias-scrollbar-hover-l1);
 }
 
 /* Rail geometry (figma rail spec): 36x36 control boxes centered in the 56px

packages/client/ui-workspace/src/client/WorkspaceBrowser.module.css

@@ -207,7 +207,9 @@
   flex: 1;
   min-height: 0;
   overflow-y: auto;
-  padding-bottom: 12px;
+  /* Clears the 72px bottom fade overlay: at scroll end the last row sits
+     above the gradient instead of under it. */
+  padding-bottom: 48px;
   /* Row trailing content (the relative time, and the hover action buttons
      that replace it) sits flush against the row's 8px right padding, so an
      overlaid scrollbar covers it. Reserving the gutter keeps the bar beside