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/src/components/ReferenceNode/ReferenceNode.vue

@@ -0,0 +1,36 @@
+<script setup lang="ts">
+defineProps<{
+  node: {
+    type: 'reference'
+    id: string
+    raw: string
+  }
+  messageId?: string
+  threadId?: string
+}>()
+defineEmits(['click', 'mouseEnter', 'mouseLeave'])
+</script>
+
+<template>
+  <span
+    class="reference-node cursor-pointer text-xs rounded-md px-1.5 mx-0.5"
+    role="button"
+    tabindex="0"
+    @click="$emit('click', $event, node.id, messageId, threadId)"
+    @mouseenter="$emit('mouseEnter', $event, node.id, messageId, threadId)"
+    @mouseleave="$emit('mouseLeave', $event, node.id, messageId, threadId)"
+  >
+    {{ node.id }}
+  </span>
+</template>
+
+<style scoped>
+.reference-node {
+  background-color: hsl(var(--ms-muted));
+  color: hsl(var(--ms-muted-foreground));
+}
+
+.reference-node:hover {
+  background-color: hsl(var(--ms-secondary));
+}
+</style>

desktop/libs/markstream-vue/src/components/ReferenceNode/index.ts

@@ -0,0 +1,8 @@
+import type { App } from 'vue'
+import ReferenceNode from './ReferenceNode.vue'
+
+ReferenceNode.install = (app: App) => {
+  app.component(ReferenceNode.__name as string, ReferenceNode)
+}
+
+export default ReferenceNode