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/en/instance.ts

@@ -114,6 +114,10 @@ export const instanceMessages = {
   "instanceShell.sessionChanges.filesChanged": "{count} files changed",
   "instanceShell.sessionChanges.actions.show": "Show changes",
 
+  "instanceShell.gitChanges.loading": "Loading git changes...",
+  "instanceShell.gitChanges.empty": "No git changes yet.",
+  "instanceShell.gitChanges.deleted": "Deleted",
+
   "instanceShell.filesShell.fileListTitle": "File list",
   "instanceShell.filesShell.mobileSelectorLabel": "Select file",
   "instanceShell.filesShell.mobileSelectorEmpty": "Select a file",