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-ja.ts

@@ -19,6 +19,8 @@ export const ja: Record<string, string> = {
   files: 'ファイル',
   changesSessionEmpty: 'このセッションにはファイル操作がまだありません',
   changesRead: '読み取り',
+  changesMdReading: '閲覧',
+  changesMdRaw: '原文',
   changesWrite: '書き込み',
   changesEdit: '編集',
   changesRunning: '実行中',