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/src/persistence.ts

@@ -17,7 +17,8 @@ import {
 } from "node:fs"
 import { homedir } from "node:os"
 import { join } from "node:path"
-import { log } from "./logger.js"
+import { log } from "./logger.ts"
+import { hasCells } from "./pages.ts"
 
 const DELAY_MS = 1000
 const MAX_FILES = 50
@@ -29,10 +30,6 @@ export function defaultDataDir(): string | null {
     return dir || join(homedir(), ".next-ai-drawio")
 }
 
-/** Any cell besides the root cells "0" and "1" */
-export const hasCells = (xml: string) =>
-    /<(mxCell\b[^>]*\bid="(?![01]")|UserObject\b|object\b)/.test(xml)
-
 export class Autosaver {
     private pending = new Map<
         string,