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/CheckboxNode/CheckboxNode.vue

@@ -0,0 +1,41 @@
+<script setup lang="ts">
+defineProps<{
+  node: {
+    type: 'checkbox'
+    checked: boolean
+    raw: string
+  }
+}>()
+</script>
+
+<template>
+  <span class="checkbox-node" role="img" :aria-label="node.checked ? 'checked' : 'unchecked'">
+    <!-- Unchecked -->
+    <svg v-if="!node.checked" xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 24 24" fill="none" class="checkbox-icon checkbox-unchecked">
+      <rect x="3" y="3" width="18" height="18" rx="4" stroke="currentColor" stroke-width="2" />
+    </svg>
+    <!-- Checked -->
+    <svg v-else xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 24 24" fill="none" class="checkbox-icon checkbox-checked">
+      <rect x="3" y="3" width="18" height="18" rx="4" fill="currentColor" />
+      <path d="M9 12l2 2 4-4" stroke="hsl(var(--ms-background))" stroke-width="2.5" stroke-linecap="round" stroke-linejoin="round" />
+    </svg>
+  </span>
+</template>
+
+<style scoped>
+.checkbox-node {
+  display: inline-flex;
+  align-items: center;
+  margin-right: 0.5em;
+  vertical-align: -0.15em;
+}
+.checkbox-icon {
+  flex-shrink: 0;
+}
+.checkbox-unchecked {
+  color: hsl(var(--ms-muted-foreground) / 0.5);
+}
+.checkbox-checked {
+  color: hsl(var(--ms-info));
+}
+</style>

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

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