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/playground-markdown-paste.test.ts

@@ -0,0 +1,32 @@
+import { describe, expect, it } from 'vitest'
+import { normalizePastedMarkdownNewlines, resolveMarkdownTextareaPaste } from '../playground-shared/markdownPaste'
+
+describe('playground markdown paste helpers', () => {
+  it('converts literal \\n sequences into real line breaks', () => {
+    expect(normalizePastedMarkdownNewlines('a\\nbxxx\\n\\nc')).toBe('a\nbxxx\n\nc')
+  })
+
+  it('keeps escaped \\\\n sequences unchanged', () => {
+    expect(normalizePastedMarkdownNewlines('a\\\\nb')).toBe('a\\\\nb')
+  })
+
+  it('ignores pasted content that already contains real line breaks', () => {
+    expect(resolveMarkdownTextareaPaste({
+      value: '',
+      selectionStart: 0,
+      selectionEnd: 0,
+    }, 'a\\nb\nc')).toBeNull()
+  })
+
+  it('applies the normalized paste over the current textarea selection', () => {
+    expect(resolveMarkdownTextareaPaste({
+      value: 'beforeafter',
+      selectionStart: 6,
+      selectionEnd: 6,
+    }, 'a\\nb')).toEqual({
+      nextValue: 'beforea\nbafter',
+      selectionStart: 9,
+      selectionEnd: 9,
+    })
+  })
+})