[P1] kotlinx.serialization + suspend Retrofit; OkHttp Authenticator for token auth

[amaury901130]

Replace Gson+Moshi with kotlinx.serialization, blocking Call.execute with suspend functions, and move Devise token header handling into interceptor/Authenticator in :core:network. Rewrite ErrorUtil parsing safely.

Acceptance criteria

• Single JSON library
• No blocking network calls
• Error parsing never throws on unexpected shapes

Reference: Android Base — Modernization Audit & Recommendations — section: Target architecture