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/math.md

@@ -0,0 +1,62 @@
+# Math Rendering Options (KaTeX)
+
+This project uses KaTeX for math rendering when available. KaTeX is a peer dependency and must be installed separately.
+
+Install KaTeX:
+
+```bash
+pnpm add katex
+```
+
+Import KaTeX styles in your entry file (e.g., `main.ts`):
+
+```ts
+import 'katex/dist/katex.min.css'
+```
+
+## CDN usage (no bundler)
+
+If you load KaTeX via `<script>` tags (so `window.katex` exists), the library can use it. If KaTeX loads after the first render, reset the loader once:
+
+```ts
+import { createKaTeXWorkerFromCDN, enableKatex, setKatexLoader, setKaTeXWorker } from 'markstream-vue'
+
+// use the CDN global (UMD) on the main thread
+setKatexLoader(() => (window as any).katex)
+enableKatex(() => (window as any).katex)
+
+// optional: offload renderToString into a worker (CDN-loaded inside worker)
+const { worker } = createKaTeXWorkerFromCDN({
+  mode: 'classic',
+  katexUrl: 'https://cdn.jsdelivr.net/npm/katex@0.16.22/dist/katex.min.js',
+  mhchemUrl: 'https://cdn.jsdelivr.net/npm/katex@0.16.22/dist/contrib/mhchem.min.js',
+})
+if (worker)
+  setKaTeXWorker(worker)
+```
+
+Customize math parsing behaviour with `getMarkdown` options or `setDefaultMathOptions`:
+
+```ts
+import { getMarkdown, setDefaultMathOptions } from 'markstream-vue'
+
+setDefaultMathOptions({
+  commands: ['infty', 'perp'],
+  escapeExclamation: true,
+})
+
+const md = getMarkdown()
+```
+
+If KaTeX is not installed, the renderer will leave math content as text and still be import-safe under SSR.
+
+Important: when writing source Markdown, always use literal (escaped) backslashes
+for TeX parenthesis delimiters. Write `\\(...\\)` rather than `\(...\)` so
+the parser can reliably detect inline TeX delimiters. Unescaped `\(...\)`
+cannot be distinguished from ordinary parentheses and may not be parsed as math.
+
+```ts
+// Try snippet to demonstrate KaTeX usage
+const example = `\\(E = mc^2\\)`
+console.log(example)
+```