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-link-node-loading-state.test.ts

@@ -0,0 +1,16 @@
+import { readFileSync } from 'node:fs'
+import { resolve } from 'node:path'
+import { describe, expect, it } from 'vitest'
+
+describe('markstream-vue2 link loading state', () => {
+  it('keeps the subtle loading hint implementation in sync', () => {
+    const source = readFileSync(resolve(process.cwd(), 'packages/markstream-vue2/src/components/LinkNode/LinkNode.vue'), 'utf8')
+
+    expect(source).toContain('link-loading-indicator')
+    expect(source).toContain('const activeOpacity = props.animationOpacity ?? 0.35')
+    expect(source).toContain('const restingOpacity = Math.max(0.12, Math.min(activeOpacity * 0.5, activeOpacity))')
+    expect(source).toContain('props.animationDuration ?? 1.6')
+    expect(source).toContain('props.animationTiming ?? \'ease-in-out\'')
+    expect(source).toContain('@keyframes underlinePulse')
+  })
+})