refactor(mcp-server): make the diagram modules usable from the web app

The web app will reuse the MCP server's XML engine instead of its own
copy in lib/utils.ts, so these modules now run in the browser too.

- Relative imports end in .ts, rewritten to .js by tsc
  (rewriteRelativeImportExtensions); Next.js resolves them directly
- Every module uses the global DOMParser/XMLSerializer: native in the
  browser, linkedom in Node via installDomPolyfill. pages.ts parsed with
  linkedom but serialized with the global serializer, which throws in
  the browser
- The saxes syntax check moves to xml-syntax.ts, so the browser does not
  pull in linkedom; it now also rejects undeclared prefixes such as
  xlink:, as the browser does
- Page decompression uses pako and atob instead of node:zlib and Buffer
- hasCells moves to pages.ts, away from the file system code
- The duplicate cell id check counts UserObject/object ids
- wrapCellsInModel drops comments and text before the first cell, which
  the web app accepts today
- validateAndFixXml takes { strict: false } for diagrams with user content
- Web tests run these modules with a browser DOM (jsdom)
- saxes becomes a direct dependency of the web app

packages/mcp-server/tests/http-server.test.ts

@@ -8,7 +8,7 @@
 
 import http from "node:http"
 import { afterAll, beforeAll, describe, expect, it } from "vitest"
-import { addHistory, getHistory } from "../src/history.js"
+import { addHistory, getHistory } from "../src/history.ts"
 import {
     getState,
     requestExport,
@@ -17,7 +17,7 @@ import {
     shutdown,
     startHttpServer,
     waitForSync,
-} from "../src/http-server.js"
+} from "../src/http-server.ts"
 
 let port = 0