fix(protocol,bridge): wire nickname validation to connect (TODO-045)

Add validate_nickname() to ProtocolClient::connect() and bridge
connect(). Trim whitespace, reject empty, truncate to 30 chars.
Re-export validate_nickname from protocol and core.

crates/chanora_bridge/src/api.rs

@@ -603,6 +603,8 @@ pub async fn connect(
     nickname: String,
     password: String,
 ) -> Result<BridgeSnapshot, BridgeError> {
+    let nickname = chanora_core::validate_nickname(&nickname)
+        .map_err(|e| BridgeError::InvalidCommand(e.to_string()))?;
     let cfg = chanora_core::ConnectConfig {
         address: host,
         nickname,