feat: Initial MVP with home server identity model

- User model with Ed25519 keypair identity
- Register/login on home server (creates keypair)
- Join guest servers with signature verification
- Public endpoints for cross-server identity
- Channel and message CRUD
- JWT authentication middleware

Architecture: Home server model, no federation, client-driven sync

lib/config/config.dart

@@ -0,0 +1,14 @@
+class Config {
+  // Server settings
+  static const int port = 9870;
+  
+  // IMPORTANT: Set this to your actual server host/domain
+  // Used for generating userId: "username@serverHost"
+  static const String serverHost = 'localhost:9870';
+  static const String serverName = 'Chat Server';
+  
+  // JWT settings
+  // TODO: Move to environment variable in production
+  static const String jwtSecret = 'your_super_secret_key_change_in_production';
+  static const Duration tokenExpiry = Duration(days: 7);
+}