fix(mobile/navigation): fix conference join hang when SDK launched in background

[iospro]

Problem

When the Jitsi Meet SDK is embedded in a host app and the host app launches the SDK
programmatically in response to an incoming call notification — while the device is
locked and the screen is off — joining the conference room hangs indefinitely.

Root cause: App.native._extraInit() awaits a rootNavigationReady promise that
resolves only when NavigationContainer fires its onReady callback. When the app
runs in the background (device locked, no UI on screen), React Navigation never renders
or fires onReady, so the polling loop spins forever and the conference join never
proceeds.

A secondary issue: without Welcome Page (embedded SDK mode), initialRouteName was
screen.connecting, but when a room URL is already known at launch the navigator
should start at screen.conference.root to avoid an unnecessary intermediate screen.

Solution

App.native.tsx — detect background SDK launch using AppState.currentState !== 'active'
combined with the presence of a url prop. Skip await rootNavigationReady in this case
so the join flow proceeds immediately without waiting for UI to become visible.

RootNavigationContainer.tsx — derive initialRouteName from Redux state:
if a room is already set at mount time (meaning the SDK was started with a conference
URL), navigate directly to screen.conference.root; otherwise use screen.connecting
as before.

Behaviour

Scenario	AppState	url prop	await skipped	initialRoute
User opens app normally	active	❌	❌	connecting
User opens app via deep link	active	✅	❌	connecting → navigate
Host app starts SDK in background	background	✅	✅	conference.root

[jitsi-jenkins]

Hi, thanks for your contribution!
If you haven't already done so, could you please make sure you sign our CLA (https://jitsi.org/icla for individuals and https://jitsi.org/ccla for corporations)? We would unfortunately be unable to merge your patch unless we have that piece :(.