perf(ui): lazy-load markdown and defer diff rendering (#215)

## Summary
- lazy-load the markdown and diff render paths so they stop inflating
initial UI startup work
- move shared text rendering helpers out of the markdown path and keep
diff rendering on the deferred path
- defer the Monaco secondary viewers so the markdown and diff path no
longer keeps that work in the main bundle

## Follow-ups
- related fork follow-up: Pagecran/CodeNomad#1
- that follow-up is now independent on dev and only keeps the remaining
right panel, picker, and tool-call secondary chunking work

## Testing
- npm run typecheck --workspace @codenomad/ui
- npm run build --workspace @codenomad/ui

packages/ui/src/lib/i18n/messages/ja/instance.ts

@@ -90,6 +90,7 @@ export const instanceMessages = {
 
   "instanceShell.rightPanel.title": "ステータスパネル",
   "instanceShell.rightPanel.tabs.changes": "変更",
+  "instanceShell.rightPanel.tabs.gitChanges": "Git 変更",
   "instanceShell.rightPanel.tabs.files": "ファイル",
   "instanceShell.rightPanel.tabs.status": "ステータス",
   "instanceShell.rightPanel.tabs.ariaLabel": "右パネルのタブ",
@@ -112,6 +113,10 @@ export const instanceMessages = {
   "instanceShell.sessionChanges.filesChanged": "{count} 個のファイルが変更されました",
   "instanceShell.sessionChanges.actions.show": "変更を表示",
 
+  "instanceShell.gitChanges.loading": "Git の変更を読み込み中...",
+  "instanceShell.gitChanges.empty": "Git の変更はまだありません。",
+  "instanceShell.gitChanges.deleted": "削除済み",
+
   "instanceShell.filesShell.fileListTitle": "ファイル一覧",
   "instanceShell.filesShell.mobileSelectorLabel": "ファイルを選択",
   "instanceShell.filesShell.mobileSelectorEmpty": "ファイルを選択してください",