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

tests/unit/utils.test.ts

@@ -1,11 +1,5 @@
 import { describe, expect, it } from "vitest"
-import {
-    applyDiagramOperations,
-    cn,
-    extractCompleteMxCells,
-    isMxCellXmlComplete,
-    wrapWithMxFile,
-} from "@/lib/utils"
+import { cn, extractCompleteMxCells, isMxCellXmlComplete } from "@/lib/utils"
 
 describe("isMxCellXmlComplete", () => {
     it("returns false for empty/null input", () => {
@@ -71,36 +65,6 @@ describe("isMxCellXmlComplete", () => {
     })
 })
 
-describe("wrapWithMxFile", () => {
-    it("wraps empty string with default structure", () => {
-        const result = wrapWithMxFile("")
-        expect(result).toContain("<mxfile>")
-        expect(result).toContain("<mxGraphModel>")
-        expect(result).toContain('<mxCell id="0"/>')
-        expect(result).toContain('<mxCell id="1" parent="0"/>')
-    })
-
-    it("wraps raw mxCell content", () => {
-        const xml = '<mxCell id="2" value="Hello"/>'
-        const result = wrapWithMxFile(xml)
-        expect(result).toContain("<mxfile>")
-        expect(result).toContain(xml)
-        expect(result).toContain("</mxfile>")
-    })
-
-    it("returns full mxfile unchanged", () => {
-        const fullXml =
-            '<mxfile><diagram name="Page-1"><mxGraphModel></mxGraphModel></diagram></mxfile>'
-        const result = wrapWithMxFile(fullXml)
-        expect(result).toBe(fullXml)
-    })
-
-    it("handles whitespace in input", () => {
-        const result = wrapWithMxFile("   ")
-        expect(result).toContain("<mxfile>")
-    })
-})
-
 describe("cn (class name utility)", () => {
     it("merges class names", () => {
         expect(cn("foo", "bar")).toBe("foo bar")
@@ -132,50 +96,3 @@ describe("extractCompleteMxCells", () => {
         )
     })
 })
-
-describe("applyDiagramOperations with wrapped cells", () => {
-    const xml = `<mxfile><diagram id="p1"><mxGraphModel><root><mxCell id="0"/><mxCell id="1" parent="0"/><UserObject id="5" label="Docs" link="https://example.com"><mxCell vertex="1" parent="1"><mxGeometry as="geometry"/></mxCell></UserObject><mxCell id="6" value="B" vertex="1" parent="1"><mxGeometry as="geometry"/></mxCell><mxCell id="e1" edge="1" parent="1" source="5" target="6"><mxGeometry relative="1" as="geometry"/></mxCell></root></mxGraphModel></diagram></mxfile>`
-
-    it("deletes a wrapped cell and its edges", () => {
-        const { result, errors } = applyDiagramOperations(xml, [
-            { operation: "delete", cell_id: "5" },
-            { operation: "delete", cell_id: "e1" },
-        ])
-        expect(errors).toEqual([])
-        expect(result).not.toContain("UserObject")
-        expect(result).not.toContain('id="e1"')
-        expect(result).toContain('id="6"')
-    })
-
-    it("rejects adding a cell with the id of a wrapped cell", () => {
-        const { errors } = applyDiagramOperations(xml, [
-            {
-                operation: "add",
-                cell_id: "5",
-                new_xml: '<mxCell id="5" vertex="1" parent="1"/>',
-            },
-        ])
-        expect(errors[0]?.message).toContain("already exists")
-    })
-
-    it("updates a wrapped cell", () => {
-        const { result, errors } = applyDiagramOperations(xml, [
-            {
-                operation: "update",
-                cell_id: "5",
-                new_xml:
-                    '<UserObject id="5" label="New" link="https://example.org"><mxCell vertex="1" parent="1"><mxGeometry as="geometry"/></mxCell></UserObject>',
-            },
-        ])
-        expect(errors).toEqual([])
-        expect(result).toContain('label="New"')
-        expect(result).not.toContain('label="Docs"')
-    })
-
-    it("reports deleting a cell that does not exist", () => {
-        const { errors } = applyDiagramOperations(xml, [
-            { operation: "delete", cell_id: "missing" },
-        ])
-        expect(errors[0]?.message).toContain("not found")
-    })
-})