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/docs/.vitepress/theme/composables/useDark.ts

@@ -0,0 +1,21 @@
+import { useData } from 'vitepress'
+
+/**
+ * VitePress theme composable for dark mode
+ * Uses VitePress's built-in isDark from useData()
+ *
+ * @example
+ * ```vue
+ * <script setup>
+ * const isDark = useDark()
+ * </script>
+ *
+ * <template>
+ *   <MarkdownRender :is-dark="isDark" />
+ * </template>
+ * ```
+ */
+export function useDark() {
+  const { isDark } = useData()
+  return isDark
+}