feat(files): tree rename (inline) + delete (confirmed) + submenu visual tether

资源管理器补全：文件树右键菜单新增「重命名」（行内编辑：预填+全选、
Enter/失焦提交、Esc 取消、IME 组合键守卫）与「删除」（Modal 确认，文件/
目录文案区分，永久删除——宿主无回收站）。根行（工作区根）不提供两项，
服务端 fs.rename/fs.remove 双保险拒绝；目标已存在 409；link-aware——
symlink 行只改/删链接本身（lstat 判定），fence 校验对解析后目标进行。

落定收尾：pruneTree 清旧前缀缓存 + 收起 stale expanded + 单层重载父目录；
tab 调解走 service（rename→updateTab 改路径改标题、delete→closeTab 关
受影响 tab，覆盖两侧分栏/底栏/浮动窗口，经 tree-mutations.ts）。

二级菜单视觉关系（layout.css，仍限 data-dsh-sidebar-submenu 作用域）：
父行在子菜单打开期间保持高亮（宿主已渲染 aria-expanded="true"），
卡片间距 10px→4px（悬停走廊同步收窄），翻转规则一并 4px。

i18n 新增 7 key × 20 语言文件（locales.spec 的 key-set-parity 门禁）。
测试：fs-operations 追加 12 例（rename/remove 全失败面）；新增
file-tree-rename-delete.spec.tsx 8 例（菜单可见性/重命名流/删除流/错误条）；
menu-flip.spec.tsx 契约扩展覆盖 tether 规则。设计文档
docs/plans/2026-09-04-file-tree-rename-delete-design.md。

src/client/api.ts

@@ -278,6 +278,15 @@ export const api = {
     call<FsTextResult | FsBinaryResult>('fs.read', scopePayload(scope, { path }), signal),
   fsWrite: (scope: SessionScope, path: string, content: string) =>
     call<{ ok: true }>('fs.write', scopePayload(scope, { path, content })),
+  /** Rename one tree row within its directory (single-segment name; the
+   *  server refuses existing destinations, the workspace root, and — while
+   *  the fence is armed — anything resolving outside the workspace). */
+  fsRename: (scope: SessionScope, path: string, name: string) =>
+    call<{ path: string }>('fs.rename', scopePayload(scope, { path, name })),
+  /** Permanently delete one tree row (recursive for directories; a symlink
+   *  row unlinks the link only). The UI confirms before calling this. */
+  fsRemove: (scope: SessionScope, path: string) =>
+    call<{ path: string }>('fs.remove', scopePayload(scope, { path })),
   /** Upload one file's raw bytes into `dir` (keeps the folder tree via
    *  `relativePath`); the host streams it under the session workspace. */
   uploadFile: (scope: SessionScope, dir: string, relativePath: string, body: Blob, signal?: AbortSignal) =>