fix: older defects (batch C) and the second batch's review

Chats:
- New Chat right after an answer saves that chat once. Saves run one at
  a time and read the chat on screen when their turn comes; a save
  scheduled for a chat that is no longer on screen is dropped. A chat
  whose id was still on its way to the URL no longer comes back after
  New Chat (the next answer went into it).
- Crossing the 768 px breakpoint keeps the chat panel: a streaming answer,
  unsaved messages and attachments stay. The panel gets the sizes of each
  side, and a panel collapsed on desktop opens on mobile.
- The chat's export waits for its own reply: an edit's history export
  still on its way no longer answers it with the older diagram, and two
  file saves at once no longer swap results.
- A second edit in one answer is previewed on the first edit's result.
- Stop also ends a running screenshot check; a chat that cannot be saved
  (storage full) can be left with "Continue without saving".
- Small diagrams with shapes count as diagrams; the tool card no longer
  crashes on malformed operations.

Quota and providers:
- Requests that reach the server's own endpoints count toward the quota:
  EdgeOne (always its own endpoint now), a private base URL whatever key
  header is sent, keyless Ollama without a URL. With the quota on, a
  redirect is followed only to a public address. The output cap applies
  to these requests too.
- Stop records the tokens of the steps that finished; the screenshot check
  counts its tokens without counting a request.
- EdgeOne configured only by AI_PROVIDER works, also in the admin Test,
  which forwards the access code. Azure set up only in the admin panel
  works in chat. The Test sends a Bedrock session token.
- The admin panel's Test of an entry without a URL uses the server's URL
  as the server does (no private address check for it); the admin panel
  no longer writes an Ollama URL.

MCP server:
- Write tools and start_session run one at a time, so two at once never
  drop each other's change; a cancelled call waiting its turn is skipped.
  get_diagram and export_diagram keep the session they started with.
- Export to .drawio first gets the user's latest edits from the browser.
- History thumbnails: one that arrives after the next AI write is
  dropped; a sync reply keeps the image; a version that changed only page
  settings is its own entry.
- A diagram over the 10 MB limit is saved without its image, or the user
  is told to download it (the server now answers 413 instead of cutting
  the connection).
- Labels holding text like id='1' or parent='1' are no longer read as
  attributes (a layer or a parent was deleted). A broken bare
  <mxGraphModel> file is refused.
- After a sync reply the tab no longer sends its autosave copy again.

Desktop and files:
- A newer switch of the same preset is not rolled back by an older one
  that failed. .env values with escaped quotes are read whole.
- MCP saved files: a file that could not be read stays protected while a
  folder without permission hides it, and is saved again once deleted.
- The desktop app reports "no chats" only when the count was read and no
  model settings are stored.

tests/unit/utils.test.ts

@@ -1,5 +1,37 @@
 import { describe, expect, it } from "vitest"
-import { cn, extractCompleteMxCells, isMxCellXmlComplete } from "@/lib/utils"
+import {
+    cn,
+    extractCompleteMxCells,
+    isMxCellXmlComplete,
+    isRealDiagram,
+} from "@/lib/utils"
+import { BLANK_MXFILE } from "@/packages/mcp-server/src/pages.ts"
+
+describe("isRealDiagram", () => {
+    it("counts a small diagram with one shape", () => {
+        // 234 characters: valid, shown, and saved with its chat
+        const xml =
+            '<mxfile><diagram id="p"><mxGraphModel><root><mxCell id="0"/><mxCell id="1" parent="0"/><mxCell id="2" value="A" vertex="1" parent="1"><mxGeometry width="80" height="30" as="geometry"/></mxCell></root></mxGraphModel></diagram></mxfile>'
+        expect(xml.length).toBeLessThan(300)
+        expect(isRealDiagram(xml)).toBe(true)
+    })
+
+    it("does not count a blank page or nothing", () => {
+        expect(isRealDiagram(BLANK_MXFILE)).toBe(false)
+        expect(isRealDiagram("")).toBe(false)
+        expect(isRealDiagram(null)).toBe(false)
+    })
+
+    it("still counts a longer document of empty named pages", () => {
+        // Pages and page settings are worth keeping, as before
+        const pages = Array.from(
+            { length: 3 },
+            (_, i) =>
+                `<diagram id="p${i}" name="Page ${i}"><mxGraphModel pageWidth="1600" pageHeight="900"><root><mxCell id="0"/><mxCell id="1" parent="0"/></root></mxGraphModel></diagram>`,
+        ).join("")
+        expect(isRealDiagram(`<mxfile>${pages}</mxfile>`)).toBe(true)
+    })
+})
 
 describe("isMxCellXmlComplete", () => {
     it("returns false for empty/null input", () => {