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/vue2-heading-node-attrs.test.ts

@@ -0,0 +1,39 @@
+/**
+ * @vitest-environment jsdom
+ */
+
+import { mount } from '@vue/test-utils'
+import { getMarkdown, parseMarkdownToStructure } from 'stream-markdown-parser'
+import { describe, expect, it } from 'vitest'
+import HeadingNode from '../packages/markstream-vue2/src/components/HeadingNode/HeadingNode.vue'
+import { flushAll } from './setup/flush-all'
+
+describe('vue2 - HeadingNode attrs passthrough', () => {
+  it('supports parseOptions.preTransformTokens injecting id on heading_open', async () => {
+    const md = getMarkdown('vue2-heading-node-attrs-test')
+    const nodes = parseMarkdownToStructure('# Hello', md, {
+      preTransformTokens(tokens: any[]) {
+        for (const token of tokens) {
+          if (token?.type !== 'heading_open')
+            continue
+          if (typeof token.attrSet === 'function')
+            token.attrSet('id', 'hello-anchor')
+          else
+            token.attrs = [...(token.attrs ?? []), ['id', 'hello-anchor']]
+        }
+        return tokens
+      },
+    })
+
+    const node = nodes[0] as any
+
+    const wrapper = mount(HeadingNode, {
+      props: {
+        node,
+      },
+    })
+
+    await flushAll()
+    expect(wrapper.get('h1.heading-node').attributes('id')).toBe('hello-anchor')
+  })
+})