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-empty-delimiter.test.ts

@@ -0,0 +1,45 @@
+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 countNodes(nodes: unknown, type: string) {
+  let count = 0
+  walk(nodes, (node) => {
+    if (node.type === type)
+      count++
+  })
+  return count
+}
+
+describe('math inline - ignore empty "$$" delimiter', () => {
+  const md = getMarkdown('math-empty-delim')
+
+  it('keeps "$$" as text to avoid perpetual loading spinners', () => {
+    const input = 'adasd $$ dasdsa\ndasdsad'
+    const nodes = parseMarkdownToStructure(input, md)
+    expect(countNodes(nodes, 'math_inline')).toBe(0)
+    expect(JSON.stringify(nodes)).toContain('$$')
+  })
+})