Implement SMART EHR launch and extract launch context (patient, encounter, user)

Steps

1. The EHR initiates the launch by redirecting to your app's launch URL with parameters iss (FHIR base URL) and launch (opaque launch token).
2. Fetch [iss]/.well-known/smart-configuration to discover the authorization endpoint.
3. Redirect to the authorization endpoint including the launch parameter and scope containing 'launch' (not 'launch/patient'); this tells the server the launch token carries context.
4. After authorization code exchange, the token response includes context claims: patient, encounter, and/or user as top-level fields alongside access_token.
5. Use the patient value directly as the patient ID for FHIR queries; it is the logical ID of the FHIR Patient resource in context.
6. Decode the id_token (a JWT) to obtain the fhirUser claim, which is the relative URL of the Practitioner or Patient resource representing the logged-in user.