refactor(web): validate and repair diagram XML with the MCP server's engine

- Delete the web app's own copy of the XML checks and repairs from
  lib/utils.ts (1,074 lines). loadDiagram now uses the MCP server's
  validateAndFixXml without the strict checks, because the XML may hold
  the user's own diagram
- display_diagram and append_diagram prepare the model's XML with the new
  shared prepareNewDiagram, also used by the MCP create_new_diagram: wrap,
  validate strictly and auto-fix while it is still a bare model (where
  duplicate ids are renamed), then turn it into an mxfile
- The streaming preview of display_diagram no longer redraws the model's
  raw cells after the tool handler loaded the checked diagram, and drops
  a queued preview once the input is complete. That redraw lost
  auto-fixes and UserObject/object wrappers, so a linked cell lost its
  label; it also showed a second error toast
- The web repair regression tests now run against the MCP functions
- New e2e test checks the canvas content after display_diagram
- Fix the e2e upload tests, whose file input locator also matched the
  template import input

hooks/use-diagram-tool-handlers.ts

@@ -6,10 +6,14 @@ import type {
 } from "@/components/chat/ValidationCard"
 import type { ValidationResult } from "@/lib/diagram-validator"
 import { formatValidationFeedback } from "@/lib/diagram-validator"
-import { isMxCellXmlComplete, wrapWithMxFile } from "@/lib/utils"
+import { isMxCellXmlComplete } from "@/lib/utils"
+import { prepareNewDiagram } from "@/packages/mcp-server/src/new-diagram.ts"
 
 const DEBUG = process.env.NODE_ENV === "development"
 
+// display_diagram replaces the document with this one page
+const NEW_PAGE = { pageId: "page-1", pageName: "Page-1" }
+
 interface ToolCall {
     toolCallId: string
     toolName: string
@@ -173,11 +177,12 @@ NEXT STEP: Call append_diagram with the continuation XML.
         const finalXml = xml
         partialXmlRef.current = "" // Reset any partial from previous truncation
 
-        // Wrap raw XML with full mxfile structure for draw.io
-        const fullXml = wrapWithMxFile(finalXml)
-
-        // loadDiagram validates and returns error if invalid
-        const validationError = onDisplayChart(fullXml)
+        // Wrap, validate and auto-fix the model's XML like the MCP server's
+        // create_new_diagram, then load it
+        const prepared = prepareNewDiagram(finalXml, NEW_PAGE)
+        const validationError = prepared.ok
+            ? onDisplayChart(prepared.xml, true)
+            : prepared.error
 
         if (validationError) {
             console.warn("[display_diagram] Validation error:", validationError)
@@ -545,8 +550,10 @@ Start your continuation with the NEXT character after where it stopped.`,
             const finalXml = partialXmlRef.current
             partialXmlRef.current = "" // Reset
 
-            const fullXml = wrapWithMxFile(finalXml)
-            const validationError = onDisplayChart(fullXml)
+            const prepared = prepareNewDiagram(finalXml, NEW_PAGE)
+            const validationError = prepared.ok
+                ? onDisplayChart(prepared.xml, true)
+                : prepared.error
 
             if (validationError) {
                 addToolOutput({