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/test/html-block-streaming-dom-stability.test.ts

@@ -0,0 +1,41 @@
+/**
+ * @vitest-environment jsdom
+ */
+import { mount } from '@vue/test-utils'
+import { describe, expect, it } from 'vitest'
+import { nextTick } from 'vue'
+import HtmlBlockNode from '../src/components/HtmlBlockNode/HtmlBlockNode.vue'
+
+describe('htmlBlockNode streaming DOM stability', () => {
+  it('keeps table element stable while loading content grows', async () => {
+    const nodeA = {
+      type: 'html_block',
+      tag: 'table',
+      raw: '<table><tr><td>1</td></tr>',
+      content: '<table><tr><td>1</td></tr></table>',
+      loading: true,
+    } as const
+
+    const wrapper = mount(HtmlBlockNode, {
+      props: {
+        node: nodeA as any,
+      },
+    })
+
+    await nextTick()
+    const first = wrapper.find('table').element
+
+    const nodeB = {
+      ...nodeA,
+      raw: '<table><tr><td>1</td></tr><tr><td>2</td></tr>',
+      content: '<table><tr><td>1</td></tr><tr><td>2</td></tr></table>',
+      loading: true,
+    } as const
+
+    await wrapper.setProps({ node: nodeB as any })
+    await nextTick()
+
+    const second = wrapper.find('table').element
+    expect(second).toBe(first)
+  })
+})