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-vue2/src/components/Portal/Portal.vue

@@ -0,0 +1,50 @@
+<script setup lang="ts">
+import { onBeforeUnmount, onMounted, ref, watch } from 'vue-demi'
+
+const props = defineProps<{ to?: string }>()
+const rootRef = ref<HTMLElement | null>(null)
+
+function resolveTarget() {
+  if (typeof document === 'undefined')
+    return null
+  if (!props.to || props.to === 'body')
+    return document.body
+  return document.querySelector(props.to)
+}
+
+function moveToTarget() {
+  const root = rootRef.value
+  if (!root)
+    return
+  const target = resolveTarget()
+  if (target && root.parentElement !== target)
+    target.appendChild(root)
+}
+
+onMounted(() => {
+  moveToTarget()
+})
+
+watch(
+  () => props.to,
+  () => {
+    moveToTarget()
+  },
+)
+
+onBeforeUnmount(() => {
+  // Vue will remove the element from its current parent automatically.
+})
+</script>
+
+<template>
+  <div ref="rootRef" class="portal-root">
+    <slot />
+  </div>
+</template>
+
+<style scoped>
+.portal-root {
+  display: contents;
+}
+</style>

desktop/libs/markstream-vue/packages/markstream-vue2/src/components/Portal/index.ts

@@ -0,0 +1,3 @@
+import Portal from './Portal.vue'
+
+export default Portal