refactor(mcp-server): move the preview page into src/preview

The 580-line page template in http-server.ts becomes index.html,
preview.css and preview.js, copied to dist/preview by the build and
filled at request time. The rendered page is unchanged apart from the
session id and draw.io origin now coming from a small config script.
Biome skips the folder because of the {{placeholders}}, as it never
linted the old template string either.

packages/mcp-server/scripts/copy-assets.mjs

@@ -15,3 +15,8 @@ for (const file of readdirSync(libSrc)) {
         cpSync(join(libSrc, file), join(libDest, file))
     }
 }
+
+// Browser preview page (HTML, CSS and script)
+cpSync(join(pkg, "src/preview"), join(pkg, "dist/preview"), {
+    recursive: true,
+})