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/plugins/fence-streaming.test.ts

@@ -0,0 +1,30 @@
+import { getMarkdown } from 'stream-markdown-parser'
+import { describe, expect, it } from 'vitest'
+
+function extractFenceTokens(md: ReturnType<typeof getMarkdown>, src: string) {
+  const tokens = md.parse(src, {}) as any[]
+  return tokens.filter((t: any) => t.type === 'fence')
+}
+
+describe('fence streaming plugin', () => {
+  it('sets closed=true for closed ``` fences', () => {
+    const md = getMarkdown('t')
+    const src = '```ts\nconst a=1\n```\n'
+    const [f] = extractFenceTokens(md, src)
+    expect(f?.meta?.closed).toBe(true)
+  })
+
+  it('sets closed=false for unclosed ``` fences', () => {
+    const md = getMarkdown('t')
+    const src = '```ts\nconst a=1\n' // no closing fence
+    const [f] = extractFenceTokens(md, src)
+    expect(f?.meta?.closed).toBe(false)
+  })
+
+  it('supports ~~~ fences and closed detection', () => {
+    const md = getMarkdown('t')
+    const src = '~~~js\nlet x=1\n~~~\n'
+    const [f] = extractFenceTokens(md, src)
+    expect(f?.meta?.closed).toBe(true)
+  })
+})