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/BlockquoteNode/BlockquoteNode.vue

@@ -0,0 +1,66 @@
+<script setup lang="ts">
+import { computed, getCurrentInstance } from 'vue-demi'
+import { isLegacyVue26Vm } from '../../utils/vue26'
+import NodeRenderer from '../NodeRenderer'
+import LegacyNodesRenderer from '../NodeRenderer/LegacyNodesRenderer.vue'
+
+interface NodeChild {
+  type: string
+  raw: string
+  [key: string]: unknown
+}
+
+interface BlockquoteNode {
+  type: 'blockquote'
+  children: NodeChild[]
+  raw: string
+  cite?: string
+}
+
+const props = defineProps<{
+  node: BlockquoteNode
+  indexKey: string | number
+  typewriter?: boolean
+  customId?: string
+}>()
+
+defineEmits<{
+  copy: [text: string]
+}>()
+
+const instance = getCurrentInstance()
+const nestedRenderer = computed(() => {
+  const vm = instance?.proxy as any
+  return isLegacyVue26Vm(vm) ? LegacyNodesRenderer : NodeRenderer
+})
+</script>
+
+<template>
+  <blockquote class="blockquote" dir="auto" :cite="node.cite">
+    <component
+      :is="nestedRenderer"
+      :index-key="`blockquote-${props.indexKey}`"
+      :nodes="props.node.children || []"
+      :custom-id="props.customId"
+      :typewriter="props.typewriter"
+      @copy="$emit('copy', $event)"
+    />
+  </blockquote>
+</template>
+
+<style scoped>
+.blockquote {
+  font-weight: 500;
+  font-style: italic;
+  border-left: 0.25rem solid var(--blockquote-border-color,#e2e8f0);
+  quotes: "\201C" "\201D" "\2018" "\2019";
+  margin: 1.6em 0;
+  padding-left: 1em;
+}
+
+.blockquote ::v-deep .markdown-renderer {
+  content-visibility: visible;
+  contain: content;
+  contain-intrinsic-size: 0px 0px;
+}
+</style>

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

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