Refactor self-connection handling and improve message processing

- Removed self-connection detection logic from the Listener and Server, simplifying the message handling process.
- Updated the HandleMessage and handle-websocket functions to eliminate checks for self-connections, enhancing clarity and maintainability.
- Adjusted AUTH challenge logic to focus solely on blacklisted IPs, streamlining connection management.

app/listener.go

@@ -24,7 +24,6 @@ type Listener struct {
 	challenge        atomic.Bytes
 	authedPubkey     atomic.Bytes
 	startTime        time.Time
-	isSelfConnection bool      // Marker to identify self-connections
 	isBlacklisted    bool      // Marker to identify blacklisted IPs
 	blacklistTimeout time.Time // When to timeout blacklisted connections
 	// Diagnostics: per-connection counters