Reads the language from the language cookie in the current HTTP request.
Uses FullstackContext::current() to access request headers during SSR.
Returns Language::default() if no context or cookie is found.
Overwrite the global language signal. Used by the app-level persistence
layer to restore a saved preference on startup before any component
renders reading the signal.
On non-wasm targets the same binary runs both the SSR server and native
clients (desktop / mobile). Distinguish them at runtime via
FullstackContext: if a fullstack request context exists we’re handling
an HTTP render and should read the language from the request cookie; if
not we’re a native client and the language lives in the global
LANGUAGE signal — the same source of truth as web — hydrated from the
WebView’s localStorage by the app layer on startup.