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/monaco-theme-scheduler.test.ts

@@ -0,0 +1,44 @@
+import { describe, expect, it, vi } from 'vitest'
+
+describe('monaco theme scheduler', () => {
+  it('dedupes concurrent same-theme updates', async () => {
+    vi.resetModules()
+    const { scheduleMonacoThemeUpdate } = await import('../packages/markstream-react/src/components/CodeBlockNode/monacoThemeScheduler')
+
+    const resolvers: Array<() => void> = []
+    const setTheme = vi.fn(() => new Promise<void>(resolve => resolvers.push(resolve)))
+
+    const p1 = scheduleMonacoThemeUpdate('vitesse-dark', setTheme)
+    const p2 = scheduleMonacoThemeUpdate('vitesse-dark', setTheme)
+
+    expect(setTheme).toHaveBeenCalledTimes(1)
+
+    resolvers[0]?.()
+    await p1
+    await p2
+
+    expect(setTheme).toHaveBeenCalledTimes(1)
+  })
+
+  it('serializes theme updates to avoid parallel registrations', async () => {
+    vi.resetModules()
+    const { scheduleMonacoThemeUpdate } = await import('../packages/markstream-react/src/components/CodeBlockNode/monacoThemeScheduler')
+
+    const resolvers: Array<() => void> = []
+    const setTheme = vi.fn(() => new Promise<void>(resolve => resolvers.push(resolve)))
+
+    const all = scheduleMonacoThemeUpdate('vitesse-dark', setTheme)
+    scheduleMonacoThemeUpdate('vitesse-light', setTheme)
+
+    expect(setTheme).toHaveBeenCalledTimes(1)
+    resolvers[0]?.()
+
+    await Promise.resolve()
+    await Promise.resolve()
+
+    expect(setTheme).toHaveBeenCalledTimes(2)
+    resolvers[1]?.()
+
+    await all
+  })
+})