feat(desktop): add structured desktop chat app

Introduce the Electron desktop app with a split app/chat/settings structure and shared nanostore state so UI areas own their state instead of routing it through the root.

apps/desktop/src/hooks/use-mobile.ts

@@ -0,0 +1,24 @@
+import * as React from 'react'
+
+const MOBILE_BREAKPOINT = 768
+const MOBILE_BREAKPOINT_REM = MOBILE_BREAKPOINT / 16
+const ONE_PIXEL_IN_REM = 1 / 16
+
+export function useIsMobile() {
+  const [isMobile, setIsMobile] = React.useState<boolean | undefined>(undefined)
+
+  React.useEffect(() => {
+    const mql = window.matchMedia(`(max-width: ${MOBILE_BREAKPOINT_REM - ONE_PIXEL_IN_REM}rem)`)
+
+    const onChange = () => {
+      setIsMobile(window.innerWidth < MOBILE_BREAKPOINT)
+    }
+
+    mql.addEventListener('change', onChange)
+    setIsMobile(window.innerWidth < MOBILE_BREAKPOINT)
+
+    return () => mql.removeEventListener('change', onChange)
+  }, [])
+
+  return !!isMobile
+}