feat(app): 支持设置 .md 默认打开方式（docs/32）

三条链路：①tauri.conf bundle.fileAssociations 注册 md/markdown，
新版安装包重装后出现在「打开方式」候选；②文件菜单「设为 .md
默认打开方式…」：Rust 命令经 reg.exe 写 HKCU 的 ProgID
NewMarkdown.md/OpenWithProgids/SupportedTypes（免管理员、指向
当前 exe），再打开系统默认应用设置让用户确认一次（Windows 不许
程序直接抢默认）；③双击打开：单实例插件转发第二实例参数到待
打开队列+聚焦主窗口，冷启动读 env args，共用 md_files_from_args
过滤；前端 take_pending_open_files 取走逐个打开，只由 main 窗口
消费防子窗口抢文件。命令数 31→33 对账，参数过滤纯函数已抽 crate
实测，vitest 154+2、构建通过；Windows 构建与双击全流程待真机验。

src/App.tsx

@@ -18,7 +18,7 @@ const SourceEditor = lazy(() => import('./components/SourceEditor'))
 import { offsetInMarkdown, type CursorAnchor } from './editor/cursorSync'
 import { insertPlainText, insertLinkText } from './editor/paste'
 import { isImeComposing } from './utils/ime'
-import { ConfirmSaveModal, AboutModal, ShortcutsModal, LinkModal, ErrorModal , EncodingConvertModal, ImageSizeModal, ImageLinkModal } from './components/Modals'
+import { ConfirmSaveModal, AboutModal, ShortcutsModal, LinkModal, ErrorModal , EncodingConvertModal, ImageSizeModal, ImageLinkModal, InfoModal } from './components/Modals'
 import { tabsReducer, createTab, activeTab, isDirty, type DocTab, type TabsState } from './stores/tabs'
 import { countWords, countChars, extractOutline, baseName, isPathInside, type OutlineItem } from './utils/text'
 import { callUndo, callRedo, focusEditor, type CursorInfo } from './editor/editor'
@@ -29,7 +29,7 @@ import {
   pickImageFile, pickAnyFile, pickAndReadWebImage, copyToAssets, saveBytesToAssets, fileToBase64,
   statFile, createTextFile, createDirPath, renamePath, deleteToTrash, revealInExplorer, type DirEntry, type RecoveryEntry,
   pickSaveHtmlFile, pickImportFile, readFileBase64, readBinaryFile, pickAndReadWebImport,
-  gitStatus, gitInitRepo, gitSetRemote, gitSyncNow, s3UploadImage, s3UploadBase64, type GitStatusInfo,
+  gitStatus, gitInitRepo, gitSetRemote, gitSyncNow, s3UploadImage, s3UploadBase64, takePendingOpenFiles, registerFileAssociation, type GitStatusInfo,
 } from './services/platform'
 import * as store from './services/store'
 import { recoveryKey, planRecovery } from './utils/recovery'
@@ -708,11 +708,47 @@ export default function App() {
       // 审计修复：这段原先写在恢复记录的提前 return 之后，无恢复记录时永远执行不到。
       const startupFile = new URLSearchParams(window.location.search).get('file')
       if (!cancelled && startupFile) void openPath(startupFile)
+      // 双击 .md 冷启动：文件路径经启动参数进 Rust 的待打开队列（docs/32）。
+      // 只由主窗口消费——子窗口（?file= 打开的）也在跑同一套启动流程，
+      // 不拦会把队列里的文件抢去自己窗口打开。
+      if (!cancelled && tauriMode) {
+        try {
+          const { getCurrentWindow } = await import('@tauri-apps/api/window')
+          if (getCurrentWindow().label === 'main') {
+            const pending = await takePendingOpenFiles()
+            for (const p of pending) { if (!cancelled) await openPath(p) }
+          }
+        } catch { /* 队列不可用时不影响正常启动 */ }
+      }
     })()
     return () => { cancelled = true }
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [])
 
+  // 第二个文件双击转来时（单实例转发）：Rust 发事件，这里取队列打开（docs/32）
+  useEffect(() => {
+    if (!tauriMode) return
+    let unlisten: (() => void) | undefined
+    let cancelled = false
+    void (async () => {
+      try {
+        const { getCurrentWindow } = await import('@tauri-apps/api/window')
+        if (getCurrentWindow().label !== 'main') return // 只由主窗口消费（理由同冷启动）
+        const { listen } = await import('@tauri-apps/api/event')
+        const un = await listen('pending-open-files', async () => {
+          try {
+            const pending = await takePendingOpenFiles()
+            for (const p of pending) await openPath(p)
+          } catch { /* 同上 */ }
+        })
+        if (cancelled) un()
+        else unlisten = un
+      } catch { /* 事件不可用时退化为仅冷启动打开 */ }
+    })()
+    return () => { cancelled = true; unlisten?.() }
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [tauriMode])
+
   useEffect(() => {
     if (workspaceRoot) {
       store.addWorkspace(workspaceRoot)
@@ -1090,6 +1126,8 @@ export default function App() {
   /** 右键图片的「超链接」目标（图片模式链接弹窗）与「调整大小」目标（docs/31） */
   const [imageLinkTarget, setImageLinkTarget] = useState<{ tabId: string; el: HTMLElement; src: string; href: string } | null>(null)
   const [imageSizeTarget, setImageSizeTarget] = useState<{ tabId: string; el: HTMLElement; width: string | null } | null>(null)
+  /** 文件关联注册后的引导文案（docs/32） */
+  const [assocMsg, setAssocMsg] = useState<string | null>(null)
   const openEditorMenu = (e: React.MouseEvent) => {
     const target = e.target as HTMLElement
     const inEditor = sourceMode
@@ -1586,6 +1624,8 @@ export default function App() {
         { divider: true },
         { label: '上传文档图片到 S3…', disabled: !tab || sourceMode, onClick: () => void runBatchImageUpload() },
         { label: 'S3 上传设置…', onClick: () => setS3SettingsOpen(true) },
+        { divider: true },
+        { label: '设为 .md 默认打开方式…', disabled: !tauriMode, onClick: () => { void (async () => { try { setAssocMsg(await registerFileAssociation()) } catch (e) { setError(e instanceof Error ? e.message : String(e)) } })() } },
       ],
     },
     {
@@ -2091,6 +2131,7 @@ export default function App() {
           }}
         />
       )}
+      {assocMsg && <InfoModal title="文件关联" message={assocMsg} onClose={() => setAssocMsg(null)} />}
       {imageSizeTarget && (
         <ImageSizeModal
           current={imageSizeTarget.width}

src/components/Modals.tsx

@@ -260,6 +260,16 @@ export function ImageLinkModal({ src, linkHref, onSubmit, onCancel }: {
   )
 }
 
+export function InfoModal({ title, message, onClose }: { title: string; message: string; onClose: () => void }) {
+  return (
+    <Shell onCancel={onClose}>
+      <h3>{title}</h3>
+      <p>{message}</p>
+      <div className="modal-actions"><button className="primary" onClick={onClose}>知道了</button></div>
+    </Shell>
+  )
+}
+
 export function ErrorModal({ message, onClose }: { message: string; onClose: () => void }) {
   return (
     <Shell onCancel={onClose}>

src/services/platform.ts

@@ -366,6 +366,16 @@ export async function s3UploadImage(config: S3ConfigPayload, filePath: string, f
   return invoke<string>('s3_upload_image', { config, filePath, fileName })
 }
 
+/** 取走待打开文件队列：启动参数/双击第二个文件转来的 .md 路径（docs/32）。 */
+export async function takePendingOpenFiles(): Promise<string[]> {
+  return invoke<string[]>('take_pending_open_files')
+}
+
+/** 注册 .md/.markdown 文件关联并打开系统默认应用设置，返回引导文案（docs/32）。 */
+export async function registerFileAssociation(): Promise<string> {
+  return invoke<string>('register_file_association')
+}
+
 /** 上传文档内嵌的 base64 图片（已剥 data: 前缀的纯 base64），返回回填地址。 */
 export async function s3UploadBase64(config: S3ConfigPayload, base64: string, fileName: string): Promise<string> {
   return invoke<string>('s3_upload_base64', { config, base64, fileName })