feat(changes): markdown reading mode for .md op previews

The changes tab preview pane gains a Reading/Raw toggle for .md/.markdown/.mdx session ops (read, write, and edit alike). Reading mode renders the resulting document through the shared MarkdownText pass (the same stack as the editor preview): GFM tables with alignment, task lists, strikethrough, footnotes, math, and code fences, with local image destinations rewritten through the session-scoped /sidebar/file media route (rewriteLocalImageUrls). Content sources: read -> parseReadContent (envelope stripped, blank lines preserved — markdown block structure depends on them, which parseReadLines drops); write -> the written content; edit -> the reconstructed resulting document from the in-window prior content when known, else the new snippet. Ported from dsh-file-trace v0.2.0 (PR #471 lineage), re-based onto the unified changes tab (#475) and the repo shared-renderer conventions (markdownTextProps labels contract, skin tokens, 21-dictionary locale parity).

src/client/locales.ts

@@ -26,6 +26,8 @@ export const zh = {
   files: '文件',
   changesSessionEmpty: '本会话还没有文件操作',
   changesRead: '读取',
+  changesMdReading: '阅读',
+  changesMdRaw: '原文',
   changesWrite: '写入',
   changesEdit: '编辑',
   changesRunning: '执行中',
@@ -435,6 +437,8 @@ export const en: Record<keyof typeof zh, string> = {
   files: 'Files',
   changesSessionEmpty: 'No file operations in this session yet',
   changesRead: 'Read',
+  changesMdReading: 'Reading',
+  changesMdRaw: 'Raw',
   changesWrite: 'Write',
   changesEdit: 'Edit',
   changesRunning: 'running',