refactor(markdown): Make module CLI-importable

Drop the mermaid import from the markdown module. The extension only emits <pre class="mermaid">, so initialize mermaid in the app entry instead. Use explicit .js specifiers plus a bundler extensionAlias so both the NodeNext CLI build and the web build resolve the sources.

src/main.tsx

@@ -1,8 +1,13 @@
 import { render } from "solid-js/web";
 import { Router, Route } from "@solidjs/router";
+import mermaid from "mermaid";
 import App from "./App";
 import "./styles.css";
 
+// Mermaid is a runtime concern; the markdown module only emits
+// <pre class="mermaid"> placeholders. Initialize before any `mermaid.run()`.
+mermaid.initialize({ theme: "dark" });
+
 const root = document.getElementById("root");
 
 if (root) {