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/packages/markstream-react/src/i18n/useSafeI18n.ts

@@ -0,0 +1,42 @@
+function humanizeKey(key: string) {
+  const s = key.split('.').pop() || key
+  return s
+    .replace(/[_-]/g, ' ')
+    .replace(/([A-Z])/g, ' $1')
+    .replace(/\s+/g, ' ')
+    .replace(/\b\w/g, c => c.toUpperCase())
+    .trim()
+}
+
+const defaultMap: Record<string, string> = {
+  'common.copy': 'Copy',
+  'common.copied': 'Copied',
+  'common.decrease': 'Decrease',
+  'common.reset': 'Reset',
+  'common.increase': 'Increase',
+  'common.expand': 'Expand',
+  'common.collapse': 'Collapse',
+  'common.preview': 'Preview',
+  'common.source': 'Source',
+  'common.export': 'Export',
+  'common.open': 'Open',
+  'common.close': 'Close',
+  'common.zoomIn': 'Zoom in',
+  'common.zoomOut': 'Zoom out',
+  'common.resetZoom': 'Reset zoom',
+  'image.loadError': 'Image failed to load',
+  'image.loading': 'Loading image...',
+}
+
+/**
+ * Replace the entire default translation map.
+ * Consumers can call this to provide their own fallback translations (e.g. Chinese).
+ */
+export function setDefaultI18nMap(map: Record<keyof typeof defaultMap, string>) {
+  Object.assign(defaultMap, map)
+}
+
+export function useSafeI18n() {
+  const t = (key: string) => defaultMap[key] ?? humanizeKey(key)
+  return { t }
+}