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/debug/isMathLike.test.ts

@@ -0,0 +1,18 @@
+import { isMathLike } from 'stream-markdown-parser'
+import { expect, it } from 'vitest'
+
+it('recognizes \"\\boldsymbol{...}\" as math-like', () => {
+  expect(isMathLike('\\boldsymbol{\\beta}')).toBe(true)
+  expect(isMathLike('\\(C++\\)')).toBe(false)
+  expect(isMathLike('(W)')).toBe(true)
+  expect(isMathLike('\\(f^{(k)}(a)\\)')).toBe(true)
+  expect(isMathLike('\\(W^\perp\\)')).toBe(true)
+  expect(isMathLike('\\(2025/9/30 21:37:24\\)')).toBe(false)
+  expect(isMathLike('operatorname{')).toBe(true)
+  expect(isMathLike('served from /vue-markdown-icon.svg')).toBe(false)
+  expect(isMathLike('served from vue-markdown-icon.1')).toBe(false)
+  expect(isMathLike('get_time')).toBe(false)
+  expect(isMathLike('or **matrix of coefficients**')).toBe(false)
+  expect(isMathLike('Quasi-Streaming')).toBe(false)
+  expect(isMathLike('\frac')).toBe(true)
+})