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/packages/markstream-vue2/src/icon/r.svg

@@ -0,0 +1,6 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16" width="16" height="16">
+  <g fill="none" stroke-linecap="round" stroke-linejoin="round">
+    <path stroke="#838ba7" d="M13.5 9.5c.63-.7 1-1.54 1-2.43 0-2.52-2.91-4.57-6.5-4.57S1.5 4.55 1.5 7.07c0 1.9 1.65 3.53 4 4.22" />
+    <path stroke="#8caaee" d="M10.5 9.5c.4 0 .86.34 1 .7l1 3.3m-5 0v-8h3.05c.95 0 1.95 1 1.95 2s-1 2-1.95 2H7.5Z" />
+  </g>
+</svg>