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/xml-validation.test.ts

@@ -8,14 +8,15 @@
  */
 
 import { beforeAll, describe, expect, it } from "vitest"
-import { getXmlSyntaxError, installDomPolyfill } from "../src/dom.js"
+import { installDomPolyfill } from "../src/dom.ts"
+import { getXmlSyntaxError } from "../src/xml-syntax.ts"
 
 beforeAll(() => {
     installDomPolyfill()
 })
 
-import { addPageToDoc, parseMxfile, serializeMxfile } from "../src/pages.js"
-import { validateAndFixXml } from "../src/xml-validation.js"
+import { addPageToDoc, parseMxfile, serializeMxfile } from "../src/pages.ts"
+import { validateAndFixXml } from "../src/xml-validation.ts"
 
 /** Bare model with the root cells plus the given cells. */
 const model = (cells: string) =>
@@ -26,6 +27,18 @@ const model = (cells: string) =>
 const BROKEN_CELL = `<mxCell id="9" value="R&D" vertex="1" parent="1"><mxGeometry as="geometry"/></mxCell>`
 
 describe("getXmlSyntaxError", () => {
+    it("rejects an attribute prefix that was never declared", () => {
+        // The browser's DOMParser, and so draw.io, rejects it too
+        expect(getXmlSyntaxError(`<mxCell id="2" xlink:href="x"/>`)).toMatch(
+            /prefix/,
+        )
+        expect(
+            getXmlSyntaxError(
+                `<mxCell xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="x"/>`,
+            ),
+        ).toBeNull()
+    })
+
     it("accepts well-formed XML", () => {
         expect(getXmlSyntaxError(model(""))).toBeNull()
     })