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/components/diff-viewer.tsx

@@ -1,9 +1,10 @@
 import { createMemo, Show, createEffect, onCleanup } from "solid-js"
 import { DiffView, DiffModeEnum } from "@git-diff-view/solid"
+import "@git-diff-view/solid/styles/diff-view-pure.css"
 import { disableCache } from "@git-diff-view/core"
 import type { DiffHighlighterLang } from "@git-diff-view/core"
 import { ErrorBoundary } from "solid-js"
-import { getLanguageFromPath } from "../lib/markdown"
+import { getLanguageFromPath } from "../lib/text-render-utils"
 import { normalizeDiffText } from "../lib/diff-utils"
 import { setCacheEntry } from "../lib/global-cache"
 import type { CacheEntryParams } from "../lib/global-cache"
@@ -134,4 +135,4 @@ export function ToolCallDiffViewer(props: ToolCallDiffViewerProps) {
       </Show>
     </div>
   )
-}
+}