Add login route and enhance login page functionality

Added a new `/login/{**catch-all}` route in `yarp.json` for the `receiver-ui` cluster. Updated `Login.razor` with a modernized UI, including a gradient header, improved error handling, and a toggle for password visibility. Integrated `AuthService` for API-based login validation.

Refactored `Login.razor` logic to handle login outcomes (`Success`, `InvalidCode`, `NotFound`, `Error`) using the new `EnvelopeLoginResult` enum. Added `LoginEnvelopeReceiverAsync` method in `AuthService` to handle login requests via `POST /api/Auth/envelope-receiver/{key}`. Improved code maintainability and user experience.

EnvelopeGenerator.API/yarp.json

@@ -9,6 +9,14 @@
           "Methods": [ "GET", "HEAD" ]
         }
       },
+      "receiver-ui-login": {
+        "ClusterId": "receiver-ui",
+        "Order": 100,
+        "Match": {
+          "Path": "/login/{**catch-all}",
+          "Methods": [ "GET", "HEAD" ]
+        }
+      },
       "receiver-ui-sender": {
         "ClusterId": "receiver-ui",
         "Order": 100,