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/prompts/migrate-react-markdown.md

@@ -0,0 +1,25 @@
+# Migrate From react-markdown
+
+Use when you want an assistant to audit and migrate a React codebase from `react-markdown` to `markstream-react`.
+
+## 中文模板
+
+```text
+请把这个 React 项目从 react-markdown 迁移到 markstream-react。
+先审计所有 react-markdown 用法，并区分：直接替换、带自定义渲染器、plugin-heavy、security-heavy。
+先完成最安全的 renderer swap，再把 HTML tag 风格的自定义渲染迁到 node-type + scoped `setCustomComponents`。
+如果存在 rehype / remark / allow-list / skipHtml / urlTransform 之类没有 1:1 对应的行为，请明确列出，不要假装完全等价。
+只有在项目确实是流式输出场景时，再建议升级到 `nodes`。
+最后运行最小可行验证并列出剩余 TODO。
+```
+
+## English template
+
+```text
+Migrate this React project from react-markdown to markstream-react.
+Audit every react-markdown usage and classify each one as direct swap, renderer-custom, plugin-heavy, or security-heavy.
+Do the safest renderer swap first, then convert HTML-tag-style custom renderers into node-type overrides with scoped `setCustomComponents`.
+If the codebase depends on rehype, remark, allow-lists, skipHtml, or urlTransform behavior that has no 1:1 match, call that out explicitly instead of claiming parity.
+Only recommend moving to `nodes` if the app is actually streaming content.
+Run the smallest useful validation step and list remaining TODOs.
+```