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.

rsbuild.config.ts

@@ -18,6 +18,13 @@ export default defineConfig({
       },
     },
     rspack: {
+      resolve: {
+        // Allow TypeScript sources to use explicit `.js` specifiers (required
+        // by the CLI's NodeNext build) while the bundler resolves them to .ts.
+        extensionAlias: {
+          ".js": [".ts", ".tsx", ".js"],
+        },
+      },
       module: {
         rules: [
           {