v0.3.7: 修复保存竞争态 + 新增源码编辑模式

Bug修复:
  - BUG-10 修复: 保存文件时 watcher restart 触发虚假 change 事件覆盖编辑器内容
    - ipc-handlers.ts: restart watcher 在 setSelfWriting(true) 保护内执行
    - App.tsx handleReloadModified: 增加 if (tab?.isModified) return 安全守卫

新功能:
  - 新增源码(Source)视图模式，直接用 textarea 编辑原始 Markdown
  - ViewMode 扩展为 editor|preview|source
  - 工具栏增加源码按钮，快捷键 Ctrl+3
  - 内容与 WYSIWYG 编辑器共享 tabStore，切换模式不丢内容

验证: TS 0错误, ESLint 0错误(1个预存警告), 90/90测试通过

src/main/ipc-handlers.ts

@@ -69,13 +69,11 @@ export function registerIpcHandlers(
         fileWatcher.stop()
         fileWatcher.setSelfWriting(true)
         const result = await saveFileContent(data.filePath, data.content)
+        // start watcher while selfWriting is still true so any immediate 'change'
+        // event from the restart is suppressed — prevents overwriting editor content
+        fileWatcher.start(data.filePath)
         fileWatcher.setSelfWriting(false)
         state.activeFilePath = data.filePath
-        setTimeout(() => {
-          if (state.activeFilePath === data.filePath && data.filePath) {
-            fileWatcher.start(data.filePath)
-          }
-        }, 300)
         if (win && !win.isDestroyed()) {
           win.setTitle(`MarkLite - ${basename(data.filePath)}`)
         }
@@ -88,12 +86,12 @@ export function registerIpcHandlers(
         if (!saveResult.canceled) {
           fileWatcher.setSelfWriting(true)
           const result = await saveFileContent(saveResult.filePath, data.content)
-          fileWatcher.setSelfWriting(false)
           if (result.success) {
             state.activeFilePath = saveResult.filePath
             fileWatcher.start(saveResult.filePath)
             win.setTitle(`MarkLite - ${basename(saveResult.filePath)}`)
           }
+          fileWatcher.setSelfWriting(false)
           return result
         }
         return { success: false, canceled: true }
@@ -116,19 +114,14 @@ export function registerIpcHandlers(
         filters: [{ name: 'Markdown 文件', extensions: ['md'] }]
       })
       if (!result.canceled) {
-        fileWatcher.stop()
         fileWatcher.setSelfWriting(true)
         const saveResult = await saveFileContent(result.filePath, data.content)
-        fileWatcher.setSelfWriting(false)
         if (saveResult.success) {
           state.activeFilePath = result.filePath
-          setTimeout(() => {
-            if (state.activeFilePath === result.filePath) {
-              fileWatcher.start(result.filePath)
-            }
-          }, 300)
+          fileWatcher.start(result.filePath)
           win.setTitle(`MarkLite - ${basename(result.filePath)}`)
         }
+        fileWatcher.setSelfWriting(false)
         return saveResult
       }
       return { success: false, canceled: true }