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/test/normalize-katex-render-input.test.ts

@@ -0,0 +1,14 @@
+import { describe, expect, it } from 'vitest'
+import { normalizeKaTeXRenderInput } from '../src/utils/normalizeKaTeXRenderInput'
+
+describe('normalizeKaTeXRenderInput', () => {
+  it('normalizes unsupported KaTeX unit glyphs', () => {
+    expect(normalizeKaTeXRenderInput('\\text{J/(kg·℃)}')).toBe('\\text{J/(kg⋅°C)}')
+    expect(normalizeKaTeXRenderInput('\\Delta t=40℃')).toBe('\\Delta t=40°C')
+  })
+
+  it('leaves already safe math content unchanged', () => {
+    expect(normalizeKaTeXRenderInput('E=mc^2')).toBe('E=mc^2')
+    expect(normalizeKaTeXRenderInput('\\text{kg}')).toBe('\\text{kg}')
+  })
+})