artifact: apps/server/src/main/kotlin/com/correx/apps/server/protocol/ClientMessage.kt
purpose: Add ChatInput data class (sessionId: SessionId, text: String, mode: ChatMode) and ChatMode enum (CHAT, STEERING) as protocol types for user-router interaction.
depends_on: none
non_goals: Implementing WebSocket handler logic, adding server response types
done_when: ChatInput serializes and deserializes through ProtocolSerializer without errors
