Add CookieNames helper for constructing cookie names

Introduce a new static class `CookieNames` in the `DigitalData.Auth.Claims` namespace to centralize and standardize the construction of cookie names for envelope receiver tokens.

- Added `GetEnvelopeReceiverCookieName` methods to generate cookie names with or without a default cookie name.
- Updated `AuthController` to use the `CookieNames` helper for constructing cookie names, replacing direct concatenation logic.
- Improved maintainability and consistency of cookie naming conventions across the application.

DigitalData.Auth.Claims/CookieNames.cs

@@ -0,0 +1,27 @@
+namespace DigitalData.Auth.Claims
+{
+    /// <summary>
+    /// Provides helpers for building cookie names used in the DigitalData.Auth ecosystem.
+    /// </summary>
+    public static class CookieNames
+    {
+        private const string ReceiverSuffix = "SignFLOWReceiver.";
+
+        /// <summary>
+        /// Builds the cookie name for an envelope receiver token.
+        /// </summary>
+        /// <param name="defaultCookieName">The base cookie name configured in <c>AuthApiParams</c>.</param>
+        /// <param name="key">The unique envelope receiver key.</param>
+        /// <returns>A cookie name in the format <c>{defaultCookieName}SignFLOWReceiver.{key}</c>.</returns>
+        public static string GetEnvelopeReceiverCookieName(string defaultCookieName, string key)
+            => defaultCookieName + ReceiverSuffix + key;
+
+        /// <summary>
+        /// Builds the cookie name for an envelope receiver token. This overload assumes a default cookie name of "AuthToken".
+        /// </summary>
+        /// <param name="key">The unique envelope receiver key.</param>
+        /// <returns>A cookie name in the format <c>{defaultCookieName}SignFLOWReceiver.{key}</c>.</returns>
+        public static string GetEnvelopeReceiverCookieName(string key)
+            => "AuthToken" + ReceiverSuffix + key;
+    }
+}