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/src/demo/math-options-example.ts

@@ -0,0 +1,11 @@
+// import { getMarkdown, KATEX_COMMANDS } from 'stream-markdown-parser'
+
+// // Example: create a MarkdownIt instance with custom math options
+// export const mdWithMathOpts = getMarkdown({
+//   mathOptions: {
+//     commands: [...KATEX_COMMANDS, 'infty', 'perp'],
+//     escapeExclamation: false,
+//   },
+// })
+
+// export const rendered = mdWithMathOpts.render('Here is math: $infty$ and bang !')