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/scripts/debug-parse-math.ts

@@ -0,0 +1,19 @@
+import { getMarkdown } from '../src/utils/markdown/getMarkdown'
+
+const md = getMarkdown({})
+
+// The content above is intentionally escaped wrong to mimic test; instead
+// use the original test string:
+const test = '可以看作 \\(' + '\\boldsymbol{\\beta}' + '\\) 与 \\(' + 'W' + '\\) 正交的一个特例（当 \\(' + 'W' + '\\) 只由 \\(' + '\\boldsymbol{\\alpha}' + '\\) 张成时）。'
+
+console.log('INPUT:', test)
+const tokens = md.parse(test, {})
+console.log('TOKENS:')
+for (const t of tokens) {
+  console.log('type:', t.type, 'tag:', t.tag, 'content:', t.content ? t.content : '')
+  if (t.children) {
+    for (const c of t.children) {
+      console.log('  child:', c.type, c.tag, c.content ? c.content : '')
+    }
+  }
+}