Add connection start time tracking and improve logging for WebSocket lifecycle

- Introduced `startTime` field in `Listener` to log connection duration.
- Enhanced diagnostics with detailed connection statistics on close.
- Improved logging and error handling for PING/PONG and message lifecycle.
- Updated version to v0.8.7.

app/listener.go

@@ -19,6 +19,7 @@ type Listener struct {
 	req          *http.Request
 	challenge    atomic.Bytes
 	authedPubkey atomic.Bytes
+	startTime    time.Time
 	// Diagnostics: per-connection counters
 	msgCount     int
 	reqCount     int