perf(frontend): enable vite module preload (#39332)

`modulePreload: false` came in with the webpack to vite migration, when
chunks were cache-busted through import maps. With content-hashed chunks
that reason is gone. Vite's default makes every lazy import preload its
whole static dependency tree in parallel instead of the browser
discovering it one level per round trip, which renders lazy features like
mermaid, the code editor and the activity charts noticeably faster on
real-world latency without changing chunks or requests.

Assisted-by: Claude Code:claude-opus-5

vite.config.ts

@@ -254,7 +254,6 @@ export default defineConfig(commonViteOpts({
     },
   },
   build: {
-    modulePreload: false,
     manifest: true,
     rolldownOptions: {
       input: {