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/nodeComponents.test.ts

@@ -0,0 +1,44 @@
+import { beforeEach, describe, expect, it } from 'vitest'
+import { clearGlobalCustomComponents, getCustomNodeComponents, removeCustomComponents, setCustomComponents } from '../src/utils/nodeComponents'
+
+describe('nodeComponents scoped API', () => {
+  beforeEach(() => {
+    // clear any global state by removing known ids
+    try {
+      removeCustomComponents('test-scope')
+      // clear global via clearGlobalCustomComponents when needed
+    }
+    catch {
+      // ignore
+    }
+  })
+
+  it('sets and retrieves a scoped mapping', () => {
+    setCustomComponents('test-scope', { custom_node: 'MyComp' })
+    const mapping = getCustomNodeComponents('test-scope')
+    expect(mapping.custom_node).toBe('MyComp')
+  })
+
+  it('sets and retrieves a global mapping (legacy)', () => {
+    setCustomComponents({ code_block: 'MarkdownCodeBlock' })
+    const mapping = getCustomNodeComponents()
+    expect(mapping.code_block).toBe('MarkdownCodeBlock')
+  })
+
+  it('removes a scoped mapping', () => {
+    setCustomComponents('test-scope', { foo: 'bar' })
+    expect(getCustomNodeComponents('test-scope').foo).toBe('bar')
+    removeCustomComponents('test-scope')
+    expect(getCustomNodeComponents('test-scope').foo).toBeUndefined()
+  })
+
+  it('clears global mapping and disallows removing global via removeCustomComponents', () => {
+    setCustomComponents({ code_block: 'MarkdownCodeBlock' })
+    expect(getCustomNodeComponents().code_block).toBe('MarkdownCodeBlock')
+    // clear global mapping
+    clearGlobalCustomComponents()
+    expect(getCustomNodeComponents().code_block).toBeUndefined()
+    // attempting to remove global via removeCustomComponents should throw
+    expect(() => removeCustomComponents('__global__')).toThrow()
+  })
+})