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/findMatchingClose.test.ts

@@ -0,0 +1,40 @@
+import { findMatchingClose } from 'stream-markdown-parser'
+import { describe, expect, it } from 'vitest'
+
+describe('findMatchingClose', () => {
+  it('finds matching close for nested parentheses with escaped braces', () => {
+    const src = '\\((\\operatorname{span}\\\\{\\boldsymbol{\\alpha}\\\\})^\\perp\\)'
+    // search for the open after the leading '\\('
+    const openIdx = src.indexOf('\\(')
+    // start looking after the opener
+    const found = findMatchingClose(src, openIdx + 2, '\\(', '\\)')
+    expect(found).toBeGreaterThan(openIdx)
+    expect(src.slice(found, found + 2)).toBe('\\)')
+  })
+
+  it('finds matching close for nested parentheses with escaped braces -1', () => {
+    const src = '"总之，\\(\\boldsymbol{\\alpha}^T \\boldsymbol{\\beta} = 0\\)"'
+
+    // search for the open after the leading '\\('
+    const openIdx = src.indexOf('\\(')
+    // start looking after the opener
+    const found = findMatchingClose(src, openIdx + 2, '\\(', '\\)')
+    console.log({ found, openIdx })
+    expect(found).toBeGreaterThan(openIdx)
+    expect(src.slice(found, found + 2)).toBe('\\)')
+  })
+
+  it('finds matching close for $$ delimiter', () => {
+    const src = '$$ E = mc^2 $$ and more'
+    const openIdx = src.indexOf('$$')
+    const found = findMatchingClose(src, openIdx + 2, '$$', '$$')
+    expect(src.slice(found, found + 2)).toBe('$$')
+  })
+
+  it('returns -1 if no close found', () => {
+    const src = '\\( unclosed math'
+    const openIdx = src.indexOf('\\(')
+    const found = findMatchingClose(src, openIdx + 2, '\\(', '\\)')
+    expect(found).toBe(-1)
+  })
+})