fix(chat): the AI reads and draws the page the user is viewing

The model always got the first page and display_diagram replaced the whole
file with one page, so drawing on page 3 of a multi-page file deleted the
other pages (#241), and edits went to page 1 while the user looked at
another page.

Now a turn records the page the user is viewing when the message is sent.
That page's model goes to the model (also as previousXml, on regenerate and
in edit_diagram error messages), selected shapes are sent on any page,
display_diagram and append_diagram replace only that page (a drawing that
brings several pages still replaces the file), edit_diagram targets it,
and the streaming preview draws on it. Compact cells and default styles
take the layer and the edges of that page, in the preview and in
editDiagram. The editor bridge replaces the current page in place when the
other pages are unchanged, so Ctrl+Z undoes an AI change on a multi-page
file in one step; when a full load is needed, the user's page is shown
again afterwards. Version summaries and the compare dialog count the page
the change was made on.

tests/unit/utils.test.ts

@@ -1,9 +1,12 @@
+import pako from "pako"
 import { describe, expect, it } from "vitest"
 import {
     cn,
     extractCompleteMxCells,
+    extractDiagramXML,
     isMxCellXmlComplete,
     isRealDiagram,
+    replaceNodes,
 } from "@/lib/utils"
 import { BLANK_MXFILE } from "@/packages/mcp-server/src/pages.ts"
 
@@ -134,3 +137,63 @@ describe("extractCompleteMxCells", () => {
         )
     })
 })
+
+const pageBox = (id: string) => `<mxCell id="${id}" vertex="1" parent="1"/>`
+const pageModel = (cells: string) =>
+    `<mxGraphModel><root><mxCell id="0"/><mxCell id="1" parent="0"/>${cells}</root></mxGraphModel>`
+const pageOf = (id: string, cells: string) =>
+    `<diagram id="${id}" name="${id}">${pageModel(cells)}</diagram>`
+/** Ids of the shapes on a page (not the root cells) */
+const shapesOn = (xml: string, pageId: string) =>
+    Array.from(
+        new DOMParser()
+            .parseFromString(xml, "text/xml")
+            .querySelectorAll(`diagram[id="${pageId}"] mxCell`),
+    )
+        .map((cell) => cell.getAttribute("id"))
+        .filter((id) => id !== "0" && id !== "1")
+
+describe("replaceNodes", () => {
+    const two = `<mxfile>${pageOf("a", pageBox("A"))}${pageOf("b", pageBox("B"))}</mxfile>`
+
+    it("replaces the first page's cells by default", () => {
+        const out = replaceNodes(two, pageBox("N"))
+        expect(shapesOn(out, "a")).toEqual(["N"])
+        expect(shapesOn(out, "b")).toEqual(["B"])
+    })
+
+    it("replaces the given page's cells and keeps the others", () => {
+        const out = replaceNodes(two, pageBox("N"), "b")
+        expect(shapesOn(out, "a")).toEqual(["A"])
+        expect(shapesOn(out, "b")).toEqual(["N"])
+    })
+
+    it("uses the first page when the given page is not there", () => {
+        const out = replaceNodes(two, pageBox("N"), "zzz")
+        expect(shapesOn(out, "a")).toEqual(["N"])
+        expect(shapesOn(out, "b")).toEqual(["B"])
+    })
+})
+
+describe("extractDiagramXML", () => {
+    const packed = (model: string) =>
+        Buffer.from(pako.deflateRaw(encodeURIComponent(model))).toString(
+            "base64",
+        )
+    const file = `<mxfile><diagram id="a" name="a">${packed(pageModel(pageBox("A")))}</diagram><diagram id="b" name="b">${packed(pageModel(pageBox("B")))}</diagram></mxfile>`
+    const escapeAttr = (s: string) =>
+        s
+            .replace(/&/g, "&amp;")
+            .replace(/"/g, "&quot;")
+            .replace(/</g, "&lt;")
+            .replace(/>/g, "&gt;")
+    const svg = `<svg xmlns="http://www.w3.org/2000/svg" content="${escapeAttr(file)}"></svg>`
+    const dataUrl = `data:image/svg+xml;base64,${Buffer.from(svg).toString("base64")}`
+
+    it("returns the first page by default and a page by its id", () => {
+        expect(extractDiagramXML(dataUrl)).toContain('id="A"')
+        expect(extractDiagramXML(dataUrl, "b")).toContain('id="B"')
+        expect(extractDiagramXML(dataUrl, "b")).not.toContain('id="A"')
+        expect(extractDiagramXML(dataUrl, "zzz")).toContain('id="A"')
+    })
+})