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/App.tsx

@@ -11,10 +11,8 @@ import InstanceDisconnectedModal from "./components/instance-disconnected-modal"
 import InstanceShell from "./components/instance/instance-shell2"
 import { SettingsScreen } from "./components/settings-screen"
 import { InstanceMetadataProvider } from "./lib/contexts/instance-metadata-context"
-import { initMarkdown } from "./lib/markdown"
 import { initGithubStars } from "./stores/github-stars"
 
-import { useTheme } from "./lib/theme"
 import { useCommands } from "./lib/hooks/use-commands"
 import { useAppLifecycle } from "./lib/hooks/use-app-lifecycle"
 import { getLogger } from "./lib/logger"
@@ -59,7 +57,6 @@ import { openSettings } from "./stores/settings-screen"
 const log = getLogger("actions")
 
 const App: Component = () => {
-  const { isDark } = useTheme()
   const { t } = useI18n()
   const {
     preferences,
@@ -183,10 +180,6 @@ const App: Component = () => {
     }
   })
 
-  createEffect(() => {
-    void initMarkdown(isDark()).catch((error) => log.error("Failed to initialize markdown", error))
-  })
-
   createEffect(() => {
     initReleaseNotifications()
   })