feat(file-icons): 文件树按扩展名区分图标 + 对外 registerFileIcon API

- 新增内置扩展名→glyph 映射（md/媒体/pdf/json/代码/配置/数据库/压缩包，
  单色 currentColor 遵循皮肤契约，未知扩展回退 VscFile）
- ctx.betterSidebar 新增 registerFileIcon/getFileIcons/matchFileIcon
  （priority 降序，同优先级 catch-all 垫底），features 增加 'fileIcons'
- FileTree 文件行消费注册表：注册图标优先（可为彩色 ReactNode，
  工厂抛错回退内置），注册/注销即时生效
- 同步 docs/external-plugin-guide.md §7 与 tests/file-icons.spec.ts

src/client/TreePanel.tsx

@@ -19,6 +19,7 @@ import { useEffect, useRef, useState, type InputHTMLAttributes } from 'react'
 import clsx from 'clsx'
 import { IconFolderOpen16, IconRefreshOutline16 } from '@deepseek-ai/dsh-client-ui-primitives'
 import { api } from './api.ts'
+import type { BetterSidebarService } from './service.ts'
 import type { SidebarStore } from './state.ts'
 import { FileTree } from './FileTree.tsx'
 import { IconUploadOutline16 } from './icons.tsx'
@@ -70,8 +71,10 @@ export function TreePanel(props: {
   /** Full-window presentation: the panel fills its host instead of docking
    *  at a fixed width. */
   full?: boolean
+  /** The sidebar registry service (file-icon registrations; passed through to FileTree). */
+  service?: BetterSidebarService
 }) {
-  const { sessionId, cwd, store, expanded, revealed, onToggle, onOpenFile, onOpenFileNewTab, onOpenFileSide, openWithTargets, openWithPinned, openWithSsh, onOpenWith, onToggleOpenWithPin, onReferenceFile, onPathRenamed, onPathDeleted, full } = props
+  const { sessionId, cwd, store, expanded, revealed, onToggle, onOpenFile, onOpenFileNewTab, onOpenFileSide, openWithTargets, openWithPinned, openWithSsh, onOpenWith, onToggleOpenWithPin, onReferenceFile, onPathRenamed, onPathDeleted, full, service } = props
   const [query, setQuery] = useState('')
   const [results, setResults] = useState<{ matches: string[]; truncated: boolean } | null>(null)
   const [error, setError] = useState<string | null>(null)
@@ -261,6 +264,7 @@ export function TreePanel(props: {
           refreshTick={refreshTick}
           onUploadRequest={startUpload}
           busy={busy}
+          service={service}
         />
       ) : (
         <div className={css.explorerBody}>