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/override-built-in-components.md

@@ -0,0 +1,25 @@
+# Override Built-in Components
+
+Use when you want an assistant to replace built-in Markstream renderers such as `image`, `link`, `code_block`, or `mermaid`.
+
+## 中文模板
+
+```text
+请在这个仓库里覆盖 Markstream 的内置组件。
+目标节点是：[填写 image / link / code_block / mermaid / d2 / infographic / inline_code / 其他]。
+默认使用 scoped `setCustomComponents(customId, mapping)`，不要直接做全局覆盖，除非有明确理由。
+如果只是 Mermaid / D2 / infographic 需要修改，请只覆盖对应 key，不要影响所有代码块。
+保留必要 props（如 `node`、`loading`、`customId`、`isDark`），并说明这个 override 会影响哪些页面或组件。
+最后运行最小可行验证。
+```
+
+## English template
+
+```text
+Override built-in Markstream components in this repository.
+The target node types are: [fill in image / link / code_block / mermaid / d2 / infographic / inline_code / other].
+Prefer scoped `setCustomComponents(customId, mapping)` instead of a global override unless there is a clear reason not to.
+If only Mermaid, D2, or infographic behavior changes, override those specific keys instead of all code blocks.
+Preserve required props such as `node`, `loading`, `customId`, and `isDark`, and explain which pages or surfaces the override affects.
+Run the smallest useful validation step at the end.
+```