refactor(web): edit and wrap diagrams with the MCP server's code

- edit_diagram runs the MCP server's editDiagram: every new_xml is checked
  first, one cell per operation, and after the edit only the target page
  is checked, rejecting only errors this edit introduced. An unrelated
  problem elsewhere in the document no longer blocks every edit. The
  error lists each failed operation
- The streaming edit preview uses the MCP applyDiagramOperations
- Delete applyDiagramOperations (292 lines) and wrapWithMxFile from
  lib/utils.ts, and the unused hand-copied scripts/test-diagram-operations.mjs
- One blank document (BLANK_MXFILE) for the web app and the MCP preview,
  replacing four copies
- Saving a .drawio wraps a bare model with normalizeToMxfile
- The empty-diagram check uses hasCells, which also counts cells wrapped
  in a UserObject/object
- DiagramOperation is the MCP type
- The wrapped-cell and empty-diagram tests now run against the MCP code
- New e2e test: edit_diagram changes the canvas, and a failing edit
  leaves it as it was

packages/mcp-server/src/http-server.ts

@@ -40,6 +40,7 @@ import {
     updateLastHistorySvg,
 } from "./history.ts"
 import { log } from "./logger.ts"
+import { BLANK_MXFILE } from "./pages.ts"
 
 // Configurable draw.io embed URL for private deployments
 const DRAWIO_BASE_URL =
@@ -57,10 +58,6 @@ function getOrigin(url: string): string {
 
 const DRAWIO_ORIGIN = getOrigin(DRAWIO_BASE_URL)
 
-// Minimal blank diagram used to bootstrap new sessions.
-// This avoids the draw.io embed spinner (spin=1) getting stuck when no `load(xml)` is ever sent.
-const DEFAULT_DIAGRAM_XML = `<mxfile host="app.diagrams.net"><diagram id="blank" name="Page-1"><mxGraphModel><root><mxCell id="0"/><mxCell id="1" parent="0"/></root></mxGraphModel></diagram></mxfile>`
-
 // Normalize URL for iframe src - ensure no double slashes
 function normalizeUrl(url: string): string {
     // Remove trailing slash to avoid double slashes
@@ -91,7 +88,9 @@ function ensureSessionStateInitialized(sessionId: string): void {
     if (stateStore.has(sessionId)) return
 
     // Not a change worth saving: the browser fills it on its next push
-    setState(sessionId, DEFAULT_DIAGRAM_XML, undefined, false, false)
+    // A blank diagram keeps the draw.io spinner (spin=1) from waiting
+    // forever when no load(xml) is ever sent
+    setState(sessionId, BLANK_MXFILE, undefined, false, false)
 }
 
 interface SessionState {