fix(plugin-toolbar): make formatting commands atomic for undo/redo

[sky86888]

Summary / 摘要

我是李伟，在boss上得到这份作业，提交至此。

Make all toolbar formatting commands atomic so each action creates a single undo/redo entry.

让所有工具栏格式化命令支持原子操作，每个操作只需一次撤销/重做即可完全恢复。

Motivation / 背景与动机

Several formatting commands in plugin-toolbar used editor.setDocument() followed by editor.setSelection(), which creates two separate CodeMirror transactions. This means users need to press undo twice to fully revert a single formatting action — once to restore the selection and once to restore the document content.

plugin-toolbar 中的多个格式化命令使用了 editor.setDocument() + editor.setSelection() 的组合，这会创建两个独立的 CodeMirror 事务。导致用户执行一个格式化操作后，需要按两次撤销才能完全恢复（第一次撤销选区，第二次才恢复文档内容）。

• Issue: N/A
• Roadmap (docs/ROADMAP.md): P1 — Power-user features / undo/redo grouping
• OpenSpec change: N/A (internal refactor, no API change)

Changes / 变更内容

• packages/plugin-toolbar:
  • Refactored toggleWrap() to use replaceRange() instead of setDocument() + setSelection()
  • Refactored insertLink() to use replaceRange()
  • Refactored toggleHeading() to use replaceRange()
  • Refactored toggleLinePrefix() / toggleBlockquote() to use replaceRange()
  • Refactored insertCodeBlock() to use replaceRange()
  • Refactored insertImage() to use replaceRange()
  • Refactored applyTextColor() to use replaceRange()
  • Refactored applyHighlight() to use replaceRange()
  • Refactored insertHorizontalRule() to use replaceRange()
  • Added atomic undo test cases for toggleBold, insertLink, and toggleHeading

Testing / 测试

• pnpm test passes / 全绿 — 48 tests in plugin-toolbar all pass
• Affected packages build (pnpm build) / 受影响包构建通过
• New / updated vitest cases / 新增或更新的 vitest 用例：
  • toggleBold — atomic undo (2 tests)
  • insertLink — atomic undo (1 test)
  • toggleHeading — atomic undo (1 test)
• Manual UI check in electron-demo / electron-demo 手动验证：
  • Bold toggle: select text → Ctrl+B → Ctrl+Z → single undo fully restores
  • Heading toggle: cursor on line → Ctrl+2 → Ctrl+Z → single undo fully restores
  • Insert link: select text → Ctrl+K → Ctrl+Z → single undo fully restores

Compliance / 合规自检

• CLA signed — first-time contributors will be prompted automatically by the CLA bot
• AI disclosure: the functional code in this PR is not primarily generated by AI. AI assistance, if any, is described below.
  AI-assisted notes / AI 使用说明：
  AI assisted with code refactoring pattern identification and test case writing.
• New dependencies (if any) listed with license & rationale:
  
• No build artifacts committed
• No secrets / .env / personal vault data committed

Checklist / 自检清单

• Title follows Conventional Commits / 标题遵循 Conventional Commits
• Public API changes update package README / types — 无 API 变更
• Touched live-preview-table.ts → walked through the 12 Table Widget rules — 不涉及
• New capability / breaking change → OpenSpec proposal linked — 不涉及，纯内部重构
• Change aligns with project scope (GOVERNANCE.md §4)

Screenshots / Recordings · 截图或录屏 (UI changes)

N/A — No UI changes, only internal transaction behavior fix.

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