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/docs/guide/mermaid-export-demo.md

@@ -0,0 +1,121 @@
+# Mermaid export demo
+
+This repository includes a runnable playground demo that shows how to intercept the export event from `MermaidBlockNode` and upload the serialized SVG (ev.svgString).
+
+- Playground route: `/mermaid-export-demo`
+- File: `playground/src/pages/mermaid-export-demo.vue`
+
+Why use this demo?
+
+- Demonstrates how to use `setCustomComponents` to override the mermaid renderer inside `MarkdownRender`.
+- Shows how to intercept `export` and use `ev.svgString` (or `ev.svgElement`) to upload or send the SVG to an API.
+
+Try it: run the playground and visit the route `/mermaid-export-demo`.
+
+Try it locally — runnable example
+
+You can copy the following single-file component into your app or into the playground to try the export handler immediately. It demonstrates a minimal `setCustomComponents` override that intercepts the `export` event and posts the serialized SVG string to a (fake) API.
+
+```vue
+<script setup lang="ts">
+import MarkdownRender, { MermaidBlockNode, setCustomComponents } from 'markstream-vue'
+import { h } from 'vue'
+
+// minimal uploader helper (replace with real upload)
+async function uploadSvgToServer(svgString: string) {
+  // Example: POST to an API endpoint
+  // await fetch('/api/upload', { method: 'POST', body: svgString })
+  return `https://example.com/uploads/mermaid-${Date.now()}.svg`
+}
+
+setCustomComponents('my-demo', {
+  mermaid: (props: any) => h(MermaidBlockNode, {
+    ...props,
+    // intercept export events (prevent default and upload svgString)
+    onExport: async (ev: any) => {
+      ev.preventDefault()
+      const svg = ev.svgString ?? (ev.svgElement ? new XMLSerializer().serializeToString(ev.svgElement) : null)
+      if (!svg) {
+        console.warn('no svg available on export')
+        return
+      }
+      const url = await uploadSvgToServer(svg)
+      // Show uploaded URL or persist as needed
+      console.log('uploaded svg to', url)
+    },
+  })
+})
+
+const md = `
+\`\`\`mermaid
+graph LR
+  A[User] --> B[Server]
+  B --> C[Storage]
+\`\`\`
+`
+</script>
+
+<template>
+  <MarkdownRender custom-id="my-demo" :content="md" />
+</template>
+```
+
+Run this in the repo playground with:
+
+```bash
+pnpm play
+# open http://localhost:5173/mermaid-export-demo or navigate to the route in the playground
+```
+
+Live demo (embedded)
+
+You can try the demo right inside the docs site — if the demo is hosted at the public playground URL this iframe will load it (or run the playground locally and open the route):
+
+<div style="max-width:960px; margin: 1rem 0; position:relative;">
+  <iframe
+    src="https://markstream-vue.simonhe.me/mermaid-export-demo"
+    title="Mermaid export demo"
+    loading="lazy"
+    sandbox="allow-forms allow-scripts allow-popups allow-same-origin"
+    style="width:100%; height:540px; border:1px solid #e5e7eb; border-radius:8px; background:#0b0b0b"
+  ></iframe>
+
+  <!-- Helpful fallback when embedding is blocked (X-Frame-Options or CSP) -->
+  <div style="margin-top:0.5rem; font-size:0.9rem; color:var(--vp-c-default);">
+    If the demo shows a blank or black iframe, the remote site may disallow embedding (X-Frame-Options/CSP) or the demo may be offline.
+    <strong>Try one of these:</strong>
+    <ul>
+      <li><a href="https://markstream-vue.simonhe.me/mermaid-export-demo" target="_blank" rel="noopener noreferrer">Open the demo in a new tab</a></li>
+      <li>Run the playground locally with <code>pnpm play</code> and visit <code>/mermaid-export-demo</code></li>
+      <li>Import the playground into CodeSandbox using the link below to edit and run the demo inline</li>
+    </ul>
+
+    <!-- Visual fallback: a representative screenshot stored in docs public assets -->
+    <div style="margin-top:0.75rem; border:1px solid rgba(0,0,0,0.06); border-radius:6px; overflow:hidden; max-width:680px;">
+      <img src="/screenshots/mermaid-demo.svg" alt="Mermaid export demo screenshot" style="width:100%; display:block; background:#fff" />
+      <div style="padding:0.5rem 0.75rem; font-size:0.85rem; color:var(--vp-c-muted);">Preview: Mermaid export demo (run locally if the iframe is blocked)</div>
+    </div>
+  </div>
+</div>
+
+Open in CodeSandbox
+
+If you'd like to edit this demo in a web-based environment, import the repo into CodeSandbox (GitHub import) and open the `playground` folder. Example URL (edit as needed):
+
+[Open the playground in CodeSandbox (GitHub import)](https://codesandbox.io/s/github/Simon-He95/markstream-vue/tree/main/playground)
+
+CodeSandbox (live editable)
+
+<div style="max-width:960px; margin: 1rem 0;">
+  <iframe
+    src="https://codesandbox.io/embed/github/Simon-He95/markstream-vue/tree/main/playground?autoresize=1&fontsize=14&hidenavigation=1"
+    title="Playground (CodeSandbox)"
+    style="width:100%; height:520px; border:1px solid #e5e7eb; border-radius:8px;"
+    sandbox="allow-modals allow-forms allow-popups allow-scripts allow-same-origin allow-popups-to-escape-sandbox"
+  ></iframe>
+
+  <div style="margin-top:0.5rem; font-size:0.9rem; color:var(--vp-c-muted);">
+    Edit and run the playground in CodeSandbox — open in a new tab if you prefer the full editor experience:
+    <a href="https://codesandbox.io/s/github/Simon-He95/markstream-vue/tree/main/playground" target="_blank" rel="noopener noreferrer">Open playground in CodeSandbox</a>
+  </div>
+</div>