* fix(desktop): handle deep link URLs from process.argv on Linux/Windows cold launch
On macOS, Electron receives plane:// deep links via the 'open-url' event.
On Linux/Windows, when the app is not running, xdg-open/shell launches the
app with the URL as a command-line argument. Without reading process.argv,
the deep link was silently dropped and Google OAuth sign-in would stall
after the browser redirect.
* docs: add Linux desktop app setup guide with protocol handler instructions
