refactor(client): 收敛 /sidebar envelope 解析与取扩展名双实现

- api.ts：call 与 fetchUpload 里逐字相同的响应 envelope 解析
  （json().catch(null) + ok/value 判断 + SidebarApiError 构造）收敛为
  私有 readEnvelope<T>()，两处调用；判断顺序与错误码回退逻辑不变。
- 取扩展名双实现收敛：extOf 的实现迁至零依赖的 paths.ts（lang.ts 与
  service.ts 均可达的基础模块），lang.ts re-export 保持既有导入面
  （tests/file-types.spec.ts 从 lang.ts 导入 extOf）；service.ts 删除
  模块私有的同体 extOfPath，matchFileViewer 改用 extOf。方向说明：
  service.ts 不能反向 import lang.ts——lang.ts 携带全部 @codemirror/*
  依赖且只经 editor 懒加载 chunk 进入浏览器，core bundle 静态引用它
  会把 CodeMirror 拉进核心包（构建产物已验证：client.js /
  client-registry.js 中 0 处 codemirror，client-editor.js 不变）。

src/client/service.ts

@@ -27,6 +27,7 @@ import {
   type SidebarSnapshot, type SidebarState, type SidebarStore, type SidebarTab,
 } from './state.ts'
 import { isNarrowWidth } from './breakpoints.ts'
+import { extOf } from './paths.ts'
 import type { SessionScope } from './api.ts'
 import type { SidebarPrefs } from '../prefs-shared.ts'
 
@@ -427,14 +428,6 @@ export interface BetterSidebarService {
   openFile(scope: SessionScope, path: string, title?: string): void
 }
 
-/** Extract the lowercase extension without leading dot from a path. */
-function extOfPath(path: string): string {
-  const at = path.lastIndexOf('.')
-  if (at === -1) return ''
-  const base = path.slice(at + 1).toLowerCase()
-  return base.includes('/') || base.includes('\\') ? '' : base
-}
-
 /** The file name of a path (both separators). */
 function baseNameOf(path: string): string {
   const at = Math.max(path.lastIndexOf('/'), path.lastIndexOf('\\'))
@@ -571,7 +564,7 @@ export function createBetterSidebarService(store: SidebarStore): BetterSidebarSe
   const isViewerEnabled = (id: string): boolean => store.getPrefs().viewersEnabled[id] !== false
 
   const matchFileViewer = (path: string, head?: Uint8Array): FileViewerDescriptor | undefined => {
-    const ext = extOfPath(path)
+    const ext = extOf(path)
     // Single pass in priority order (descending; stable for equal
     // priorities — insertion order). Each descriptor gets first refusal in
     // its own turn: `detect` (when head bytes are available) beats its own