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/math-inline-escaped-parens-duplicate.test.ts

@@ -0,0 +1,48 @@
+import { getMarkdown, parseMarkdownToStructure } from 'stream-markdown-parser'
+import { describe, expect, it } from 'vitest'
+
+function walk(value: unknown, visit: (node: any) => void, seen = new WeakSet<object>()) {
+  if (!value)
+    return
+  if (typeof value !== 'object')
+    return
+  if (seen.has(value))
+    return
+  seen.add(value)
+
+  if (Array.isArray(value)) {
+    for (const item of value)
+      walk(item, visit, seen)
+    return
+  }
+
+  const node = value as any
+  if (typeof node.type === 'string')
+    visit(node)
+
+  for (const child of Object.values(node))
+    walk(child as any, visit, seen)
+}
+
+function collectText(nodes: unknown) {
+  const parts: string[] = []
+  walk(nodes, (node) => {
+    if (node.type === 'text' && typeof node.content === 'string')
+      parts.push(node.content)
+  })
+  return parts.join('')
+}
+
+describe('math inline - avoid duplicated text around \\( \\)', () => {
+  const md = getMarkdown('math-inline-escaped-parens-duplicate')
+
+  it('does not duplicate the text span when a \\( \\) pair is skipped as non-math', () => {
+    const input = String.raw`计算\( (\complement_{\text{R}}P)\cap M \)
+求两个集合的交集，即取同时属于\( [0,1] \)和\( \left( \frac{1}{2},+\infty \right) \)的部分，得\\( \left( \frac{1}{2},1 \right] \\)，对应选项A。`
+
+    const nodes = parseMarkdownToStructure(input, md)
+    const text = collectText(nodes)
+    const needle = '求两个集合的交集，即取同时属于( [0,1] )'
+    expect(text.split(needle).length - 1).toBe(1)
+  })
+})