Add plan for recent receiver suggestions feature

Added a detailed plan document outlining the development of a "recent receiver suggestions" feature for step 3 of the sender wizard. The plan defines team working rules, objectives, functional requirements, and architecture direction across application, server, and client layers.

The implementation is divided into six phases, covering MediatR query creation, API endpoint addition, client integration, UI updates, regression testing, and technical debt tracking. Each phase includes black-box test suggestions to ensure quality and robustness. The plan emphasizes incremental development, user approval, and quality-focused processes.

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Controllers/EnvelopeReceiverController.cs

@@ -112,6 +112,20 @@ public class EnvelopeReceiverController : ControllerBase
         return name is null ? NotFound() : Ok(name);
     }
 
+    /// <summary>
+    /// Returns recent receiver suggestions for the authenticated sender.
+    /// </summary>
+    /// <param name="query">Suggestion query options (take 3..5 is applied in handler).</param>
+    /// <param name="cancel">Cancellation token.</param>
+    /// <returns>List of recent distinct receiver suggestions.</returns>
+    [Authorize(AuthenticationSchemes = AuthScheme.Sender)]
+    [HttpGet("recent-suggestions")]
+    public async Task<IActionResult> GetRecentSuggestions([FromQuery] ReadRecentReceiverSuggestionsQuery query, CancellationToken cancel)
+    {
+        var result = await _mediator.Send(query.ForSender(User.GetUsername()), cancel);
+        return Ok(result);
+    }
+
     /// <summary>
     /// Datenübertragungsobjekt mit Informationen zu Umschlägen, Empfängern und Unterschriften.
     /// </summary>
@@ -278,4 +292,4 @@ public class EnvelopeReceiverController : ControllerBase
         }
     }
 
-}
+}