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/SUPPORT.md

@@ -0,0 +1,29 @@
+# Support
+
+Need help with markstream-vue? Start here.
+
+## Self-serve
+
+- Docs: https://markstream-vue-docs.simonhe.me/guide/
+- 中文文档: https://markstream-vue-docs.simonhe.me/zh/guide/
+- Troubleshooting: https://markstream-vue-docs.simonhe.me/guide/troubleshooting
+- Playground (interactive demo): https://markstream-vue.simonhe.me/
+- Test page (shareable repro): https://markstream-vue.simonhe.me/test
+
+## Ask a question
+
+- GitHub Discussions: https://github.com/Simon-He95/markstream-vue/discussions
+- Discord: https://discord.gg/vkzdkjeRCW
+
+## File a bug
+
+- Use the bug template and attach a repro link from the test page: https://github.com/Simon-He95/markstream-vue/issues/new?template=bug_report.yml
+- Include the exact Markdown/content and versions (markstream-vue, Vue, browser/runtime).
+
+## Feature requests
+
+- Use the feature template and describe the use case (AI/chat/SSE, docs, diff viewer, custom nodes): https://github.com/Simon-He95/markstream-vue/issues/new?template=feature_request.yml
+
+## Security
+
+- Please report privately via [SECURITY.md](./SECURITY.md) (email: simon_he@outlook.com).