feat(editor): sync changes to local Markdown files

[RerankerGuo]

Summary

Closes #24.

CodeCV currently keeps editor changes in localStorage, so browser content
can diverge from a local Markdown source file. This PR adds an opt-in local
file binding that writes later editor changes back to a user-approved .md
file.

Changes

• add a Sync local MD action to the editor header;
• request a Markdown file through the File System Access API;
• require an explicit choice when the file and editor content differ;
• debounce and serialize later writes to the active file handle;
• abort failed write streams and prevent pending writes from crossing file
  bindings;
• keep the existing import/export workflow for unsupported browsers.

The browser still cannot discover or write arbitrary local paths. Users must
select a file, and the binding lasts for the current page session.

Verification

• npx prettier --check src/views/editor/hook.ts src/views/editor/editor.vue src/views/editor/components/header/header.vue
• npx eslint src/views/editor/hook.ts src/views/editor/editor.vue src/views/editor/components/header/header.vue
• npm run build
• browser regression with a controlled File System Access handle:
  • editor content overwrote the selected file only after confirmation;
  • selecting the local-file version replaced the editor and preview;
  • a later editor change produced one debounced write and one close;
  • editor, preview, and written content contained the same marker;
  • the preview rendered one page.

The current master branch has the unrelated duplicate-route-name startup
failure covered by #22. Browser verification applied those three route
renames temporarily; they are not included in this PR.

Compatibility

• Chromium-based browsers in a secure context (including localhost) can use
  local synchronization.
• Other browsers receive a warning and can continue using Import MD / Export
  MD.

[RerankerGuo]

@acmenlei 您好，抱歉打扰。

我们在本地使用 CodeCV 时发现，浏览器编辑后的 Markdown 只保存在 localStorage，无法与本地 .md 文件同步，容易造成两份内容分叉，因此提交了 #24 和这个最小范围的 PR。

本 PR 仅修改编辑器相关的 3 个文件，新增用户主动授权后的本地文件同步，并保留原有导入/导出作为兼容回退；已通过 Prettier、ESLint、npm run build 以及浏览器双向同步回归。您方便时烦请看一下这个方向是否符合项目预期；如需调整，我会及时跟进。感谢您维护 CodeCV。