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.

biome.json

@@ -7,7 +7,7 @@
     },
     "files": {
         "ignoreUnknown": false,
-        "includes": ["**", "!public"]
+        "includes": ["**", "!public", "!packages/mcp-server/src/preview"]
     },
     "formatter": {
         "enabled": true,