fix(ui): compact context menus + viewport-clamped submenu flip

减重：全部右键菜单（FileTree 行菜单 / TabBar tab 菜单 / GitLens 文件与
历史菜单 / FreeWindow 标题菜单）与「+」菜单切到宿主 Menu 原语的 compact
密度（26px 行高 / 12px 字 / 164px 卡片），图标随槽位 16→14，「打开方式」
子菜单图钉 20→16，还原工具型右键菜单的趁手感。

越界：宿主子菜单纯 CSS 定位（从父行向上、向右生长）不受视口钳制——树顶
部右键时「打开方式 ▸」子菜单伸出视口上沿，窄面板（280–400px）时伸出右
沿。新增 menu-flip.ts：菜单打开期间在 <body> 发布 data-dsh-sidebar-submenu
="down|left" 方向 token（y < vh/2 → down；x + 400 > vw → left），layout.css
据此翻转子菜单生长方向与 ::before 悬停桥；属性存续期即作用域，宿主自有
菜单不受影响。刻意不做 max-height 滚动钳制——overflow 裁剪会切断悬停桥。

测试：menu-flip.spec.tsx（token 纯函数 + hook 生命周期 + CSS 契约跨文件
一致性）；tab-bar / file-tree 两个 spec 追加翻转向量断言；mount.e2e 追加
compact 行高（≤30px）布局级证据。设计偏差记录同步至
docs/plans/2026-08-22-open-with-menu-design.md。

src/client/layout.css

@@ -111,3 +111,40 @@ body[data-dsh-sidebar-dragging] #root [data-dsh-center-col],
     transition: none;
   }
 }
+
+/* Context-menu submenu flip (viewport clamping for the portaled menus).
+ *
+ * The ui-primitives Menu clamps its main portal list into the viewport but
+ * positions nested submenus with static CSS only — anchored at the parent
+ * row's bottom, growing UP and to the RIGHT — with no clamp whatsoever.
+ * Docked on the window's right edge, both defaults are the worst case for
+ * this panel: a tall "open with" submenu escapes past the viewport top when
+ * the row menu opens near the top, and past the right edge on narrow panels
+ * (PANEL_MIN 280 < card 218 + gap + submenu 165).
+ *
+ * menu-flip.ts publishes `data-dsh-sidebar-submenu` on <body> — with the
+ * words "down" and/or "left" — for exactly the lifetime of one of OUR
+ * submenu-bearing menus (FileTree row menu, TabBar tab menu). The tokens
+ * flip the growth direction; the attribute's absence restores the
+ * primitive's own geometry untouched, and host-owned menus are never
+ * affected because they never open while the attribute exists (each side
+ * closes on the other's outside pointerdown).
+ *
+ * No max-height/overflow scroll cap here on purpose: overflow clipping
+ * would cut the submenu's ::before hover bridge (the 10px corridor that
+ * keeps itemWrap's mouseleave from closing the card mid-crossing). */
+body[data-dsh-sidebar-submenu~="down"] div[role="menu"] div[role="menu"] {
+  top: -4px;
+  bottom: auto;
+}
+
+body[data-dsh-sidebar-submenu~="left"] div[role="menu"] div[role="menu"] {
+  left: auto;
+  right: calc(100% + 10px);
+}
+
+/* The hover bridge follows the card to the left side of the parent row. */
+body[data-dsh-sidebar-submenu~="left"] div[role="menu"] div[role="menu"]::before {
+  left: auto;
+  right: -10px;
+}