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/packages/markstream-react/src/server-renderer/html.tsx

@@ -0,0 +1 @@
+export { parseHtmlToReactNodes } from '../utils/htmlToReact'

desktop/libs/markstream-vue/packages/markstream-react/src/server-renderer/katex.ts

@@ -0,0 +1,74 @@
+import { normalizeKaTeXRenderInput } from '../utils/normalizeKaTeXRenderInput'
+
+let katexModule: any | undefined
+let katexLoadAttempted = false
+
+function getRequire() {
+  try {
+    const processWithBuiltins = Reflect.get(globalThis, 'process') as {
+      getBuiltinModule?: (specifier: string) => any
+    } | undefined
+    const moduleApi = typeof processWithBuiltins?.getBuiltinModule === 'function'
+      ? processWithBuiltins.getBuiltinModule('module')
+      : null
+    return typeof moduleApi?.createRequire === 'function'
+      ? moduleApi.createRequire(import.meta.url)
+      : null
+  }
+  catch {
+    return null
+  }
+}
+
+function loadKatex() {
+  if (katexModule)
+    return katexModule
+  if (katexLoadAttempted)
+    return null
+
+  katexLoadAttempted = true
+
+  try {
+    const require = getRequire()
+    if (!require)
+      return null
+    const katex = require('katex')
+    try {
+      require('katex/contrib/mhchem')
+    }
+    catch {
+      // optional extension
+    }
+    katexModule = katex
+    return katexModule
+  }
+  catch {
+    return null
+  }
+}
+
+export function renderKatexToHtml(
+  content: string,
+  displayMode: boolean,
+  throwOnError: boolean,
+) {
+  const normalizedContent = normalizeKaTeXRenderInput(content)
+  if (!normalizedContent)
+    return null
+  if ((globalThis as any).__MARKSTREAM_REACT_DISABLE_SYNC_KATEX__)
+    return null
+
+  const katex = loadKatex()
+  if (!katex)
+    return null
+
+  try {
+    return katex.renderToString(normalizedContent, {
+      displayMode,
+      throwOnError,
+    })
+  }
+  catch {
+    return null
+  }
+}