Add pages for sender/receiver login and homepage UI

Added `EnvelopeSenderPage.razor` as a placeholder for the sender's dashboard.
Updated `Index.razor` to include a homepage with a hero header, feature badges,
and dynamic description rendering using JavaScript interop.

Implemented `LoginReceiverPage.razor` for secure document access via access code,
with error handling and user feedback for various login states.

Implemented `LoginSenderPage.razor` for sender authentication, including error
handling, password visibility toggle, and redirection to the sender dashboard.

EnvelopeGenerator.WebUI/EnvelopeGenerator.WebUI.Client/Pages/EnvelopeSenderPage.razor

@@ -0,0 +1,8 @@
+@page "/sender"
+@rendermode InteractiveWebAssembly
+
+<h3>EnvelopeSender</h3>
+
+@code {
+
+}