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/instance/shell/right-panel/tabs/ChangesTab.tsx

@@ -1,11 +1,13 @@
-import { For, Show, createMemo, type Accessor, type Component, type JSX } from "solid-js"
-
-import { MonacoDiffViewer } from "../../../../file-viewer/monaco-diff-viewer"
+import { For, Show, Suspense, createMemo, lazy, type Accessor, type Component, type JSX } from "solid-js"
 
 import DiffToolbar from "../components/DiffToolbar"
 import SplitFilePanel from "../components/SplitFilePanel"
 import type { DiffContextMode, DiffViewMode, DiffWordWrapMode } from "../types"
 
+const LazyMonacoDiffViewer = lazy(() =>
+  import("../../../../file-viewer/monaco-diff-viewer").then((module) => ({ default: module.MonacoDiffViewer })),
+)
+
 interface ChangesTabProps {
   t: (key: string, vars?: Record<string, any>) => string
 
@@ -113,15 +115,23 @@ const ChangesTab: Component<ChangesTabProps> = (props) => {
             }
           >
             {(file) => (
-              <MonacoDiffViewer
-                scopeKey={scopeKey()}
-                path={String(file().file || "")}
-                before={String((file() as any).before || "")}
-                after={String((file() as any).after || "")}
-                viewMode={props.diffViewMode()}
-                contextMode={props.diffContextMode()}
-                wordWrap={props.diffWordWrapMode()}
-              />
+              <Suspense
+                fallback={
+                  <div class="file-viewer-empty">
+                    <span class="file-viewer-empty-text">{props.t("instanceInfo.loading")}</span>
+                  </div>
+                }
+              >
+                <LazyMonacoDiffViewer
+                  scopeKey={scopeKey()}
+                  path={String(file().file || "")}
+                  before={String((file() as any).before || "")}
+                  after={String((file() as any).after || "")}
+                  viewMode={props.diffViewMode()}
+                  contextMode={props.diffContextMode()}
+                  wordWrap={props.diffWordWrapMode()}
+                />
+              </Suspense>
             )}
           </Show>
         </div>
@@ -220,7 +230,7 @@ const ChangesTab: Component<ChangesTabProps> = (props) => {
         onResizeMouseDown={props.onResizeMouseDown}
         onResizeTouchStart={props.onResizeTouchStart}
         isPhoneLayout={props.isPhoneLayout()}
-        overlayAriaLabel="Changes"
+        overlayAriaLabel={props.t("instanceShell.rightPanel.tabs.changes")}
       />
     )
   }