fix: resolve zero-time dashboard display and improve SQL robustness

Fixed '2025 years ago' issue by correctly handling zero-value timestamps. Improved SQL scanning logic to handle NULL values more safely across all analytics handlers.

internal/server/dashboard.go

@@ -549,10 +549,13 @@ func (s *Server) handleGetClients(c *gin.Context) {
 			desc = *cl.Description
 		}
 
+		var lastUsedTime sql.NullTime
+		_ = s.database.Get(&lastUsedTime, "SELECT MAX(last_used_at) FROM client_tokens WHERE client_id = ?", cl.ClientID)
+
 		var lastUsed *time.Time
-		err := s.database.Get(&lastUsed, "SELECT MAX(last_used_at) FROM client_tokens WHERE client_id = ?", cl.ClientID)
-		if err != nil && err != sql.ErrNoRows {
-			// ignore
+		if lastUsedTime.Valid && !lastUsedTime.Time.IsZero() {
+			t := lastUsedTime.Time
+			lastUsed = &t
 		}
 
 		uiClients[i] = UIClient{