feat(desktop): embed BoosAPI Desktop client as a subproject

Vendor the rebranded BoosAPI Desktop client (Tauri + Vue) into metapi as
a monorepo subdirectory under desktop/. Source only — node_modules,
dist, Cargo target and the upstream .git history are excluded.

desktop/ keeps its own independent Cargo workspace (desktop/src-tauri
with crates/*); it is not part of the backend boosapi crate, so a
`cargo check` at the metapi root still compiles only the backend. Build
the desktop app from desktop/src-tauri per desktop/README.md.

Note: app icons are still placeholder assets pending the final BoosAPI
logo. The updater signing keys live outside the repo and are not
included.

desktop/libs/markstream-vue/test/setup/flush-all.ts

@@ -0,0 +1,13 @@
+import { nextTick } from 'vue'
+
+/**
+ * Flushes Vue microtasks and a couple of macrotasks to allow async chains
+ * like: visibility await -> worker reject -> getKatex -> sync render -> DOM update.
+ */
+export async function flushAll() {
+  await nextTick()
+  await Promise.resolve()
+  await Promise.resolve()
+  await new Promise(r => setTimeout(r, 0))
+  await new Promise(r => setTimeout(r, 0))
+}

desktop/libs/markstream-vue/test/setup/vitest.setup.ts

@@ -0,0 +1,111 @@
+import { Buffer } from 'node:buffer'
+import { vi } from 'vitest'
+
+class WorkerStub {
+  onmessage: ((event: MessageEvent) => void) | null = null
+  onerror: ((event: ErrorEvent) => void) | null = null
+  addEventListener() {}
+  removeEventListener() {}
+  postMessage() {}
+  terminate() {}
+}
+
+if (!(globalThis as any).Worker)
+  (globalThis as any).Worker = WorkerStub as unknown as typeof Worker
+
+if (!(globalThis as any).ResizeObserver) {
+  class ResizeObserverStub {
+    observe() {}
+    unobserve() {}
+    disconnect() {}
+  }
+  ;(globalThis as any).ResizeObserver = ResizeObserverStub
+}
+
+if (!(globalThis as any).btoa) {
+  ;(globalThis as any).btoa = (input: string) => Buffer.from(input, 'utf-8').toString('base64')
+}
+
+if (!(globalThis as any).atob) {
+  ;(globalThis as any).atob = (input: string) => Buffer.from(input, 'base64').toString('utf-8')
+}
+
+const editorView = {
+  getModel: () => ({ getLineCount: () => 1 }),
+  getOption: () => 14,
+  updateOptions: () => {},
+  layout: () => {},
+}
+
+const diffEditorView = {
+  getModel: () => ({ getLineCount: () => 1 }),
+  getOption: () => 14,
+  updateOptions: () => {},
+  layout: () => {},
+}
+
+const streamMonacoHelpers = {
+  useMonaco: vi.fn(() => streamMonacoHelpers),
+  createEditor: vi.fn(async () => {}),
+  createDiffEditor: vi.fn(async () => {}),
+  updateCode: vi.fn(),
+  updateDiff: vi.fn(),
+  getEditor: vi.fn(() => null),
+  getEditorView: vi.fn(() => editorView),
+  getDiffEditorView: vi.fn(() => diffEditorView),
+  cleanupEditor: vi.fn(() => {}),
+  safeClean: vi.fn(() => {}),
+  refreshDiffPresentation: vi.fn(() => {}),
+  setTheme: vi.fn(async () => {}),
+}
+
+// Tests reach into this global handle to reset/inspect the shared stream-monaco mock.
+;(globalThis as any).__streamMonacoHelpers = streamMonacoHelpers
+
+vi.mock('stream-monaco', () => ({
+  useMonaco: streamMonacoHelpers.useMonaco,
+  preloadMonacoWorkers: vi.fn(async () => {}),
+  getOrCreateHighlighter: vi.fn(async () => ({
+    codeToTokens: vi.fn(() => ({
+      tokens: [],
+      fg: '#000000',
+      bg: '#ffffff',
+      themeName: 'vitesse-dark',
+      rootStyle: {},
+      grammarState: null,
+    })),
+  })),
+  detectLanguage: () => 'plaintext',
+}))
+
+vi.mock('stream-monaco/legacy', () => {
+  const mod = {
+    useMonaco: streamMonacoHelpers.useMonaco,
+    preloadMonacoWorkers: vi.fn(async () => {}),
+    getOrCreateHighlighter: vi.fn(async () => ({
+      codeToTokens: vi.fn(() => ({
+        tokens: [],
+        fg: '#000000',
+        bg: '#ffffff',
+        themeName: 'vitesse-dark',
+        rootStyle: {},
+        grammarState: null,
+      })),
+    })),
+    detectLanguage: () => 'plaintext',
+  }
+  return {
+    ...mod,
+    default: mod,
+  }
+})
+
+vi.mock('mermaid', () => ({
+  default: {
+    initialize: () => {},
+    render: async (_id: string, code: string) => `<svg data-testid="mermaid-svg">${code}</svg>`,
+    parse: () => {},
+  },
+}))
+
+// Do not mock katexWorkerClient here to allow tests to exercise real behavior.