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/playground-vue2-cli/public/favicon.svg

@@ -0,0 +1,9 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 32 32">
+  <style>
+    path { fill: #222; }
+    @media (prefers-color-scheme: dark) {
+      path { fill: #ffffff; }
+    }
+  </style>
+  <path d="M27.562 26L17.17 8.928l2.366-3.888L17.828 4L16 7.005L14.17 4l-1.708 1.04l2.366 3.888L4.438 26H2v2h28v-2zM16 10.85L25.22 26H17v-8h-2v8H6.78z" />
+</svg>

desktop/libs/markstream-vue/playground-vue2-cli/public/index.html

@@ -0,0 +1,22 @@
+<!DOCTYPE html>
+<html lang="en">
+  <head>
+    <meta charset="utf-8" />
+    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
+    <meta name="viewport" content="width=device-width,initial-scale=1.0" />
+    <link rel="icon" href="<%= BASE_URL %>favicon.svg" type="image/svg+xml" />
+    <meta
+      name="description"
+      content="markstream-vue2 Vue CLI playground for Vue 2.6 and Webpack 4 compatibility testing."
+    />
+    <title>markstream-vue2 Playground | Vue CLI 4 / Webpack 4</title>
+  </head>
+  <body>
+    <noscript>
+      <strong>This app requires JavaScript to run.</strong>
+    </noscript>
+    <div id="app"></div>
+    <!-- Load Mermaid as a global for Webpack 4 compatibility (avoid `exports`-only ESM entry). -->
+    <script src="https://cdn.jsdelivr.net/npm/mermaid@11/dist/mermaid.min.js"></script>
+  </body>
+</html>

desktop/libs/markstream-vue/playground-vue2-cli/public/vue-markdown-icon.svg

@@ -0,0 +1,18 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 128 128" role="img" aria-label="Vue Markdown Renderer">
+  <title>Vue Markdown Renderer</title>
+  <defs>
+    <linearGradient id="g-vue" x1="0" x2="1" y1="0" y2="1">
+      <stop offset="0" stop-color="#42b883"/>
+      <stop offset="1" stop-color="#35495e"/>
+    </linearGradient>
+    <linearGradient id="g-md" x1="0" x2="1" y1="0" y2="1">
+      <stop offset="0" stop-color="#111827"/>
+      <stop offset="1" stop-color="#374151"/>
+    </linearGradient>
+  </defs>
+  <rect x="8" y="10" width="112" height="108" rx="14" fill="url(#g-md)"/>
+  <path d="M64 18L38 18L18 18L64 100L110 18L90 18L64 64L38 18Z" fill="url(#g-vue)"/>
+  <path d="M30 74h10l8 10 8-10h10v26h-9V89l-9 11-9-11v11h-9V74zm58 0h10l8 10 8-10h10v26h-9V89l-9 11-9-11v11h-9V74z" fill="#f9fafb"/>
+</svg>
+