Add LoginSender.razor for Sender user authentication

Implemented a new `LoginSender.razor` component to provide a login page for "Sender" users. The page includes:
- A responsive card layout with a header, input fields for "Username" and "Password," and a submit button with a loading spinner.
- Error handling for invalid credentials and server errors, with appropriate alerts.
- A password visibility toggle for better user experience.
- Integration with `AuthService` for authentication and `NavigationManager` for redirection.

Added Blazor code-behind logic to manage state, handle login submission, and trigger login on "Enter" key press. The page is styled with a custom theme and includes a footer for support information.

EnvelopeGenerator.ReceiverUI/Pages/LoginSender.razor

@@ -0,0 +1,172 @@
+@page "/login"
+@using EnvelopeGenerator.ReceiverUI.Services
+@inject AuthService AuthService
+@inject NavigationManager Navigation
+
+<link href="_content/DevExpress.Blazor.Themes/blazing-berry.bs5.min.css" rel="stylesheet" />
+
+<div class="login-page-wrapper d-flex align-items-center justify-content-center min-vh-100">
+    <div class="login-card card shadow border-0" style="max-width: 440px; width: 100%;">
+
+        <div class="card-header text-white text-center py-4 border-0" style="background: linear-gradient(135deg, #2c3e50 0%, #3498db 100%); border-radius: calc(0.375rem - 1px) calc(0.375rem - 1px) 0 0;">
+            <div class="mb-2">
+                <svg xmlns="http://www.w3.org/2000/svg" width="40" height="40" fill="currentColor" viewBox="0 0 16 16">
+                    <path d="M11 6a3 3 0 1 1-6 0 3 3 0 0 1 6 0z"/>
+                    <path fill-rule="evenodd" d="M0 8a8 8 0 1 1 16 0A8 8 0 0 1 0 8zm8-7a7 7 0 0 0-5.468 11.37C3.242 11.226 4.805 10 8 10s4.757 1.225 5.468 2.37A7 7 0 0 0 8 1z"/>
+                </svg>
+            </div>
+            <h5 class="mb-0 fw-semibold">Sender Anmeldung</h5>
+            <p class="mb-0 mt-1 opacity-75" style="font-size: 0.85rem;">Sicherer Zugang zum Sender-Dashboard</p>
+        </div>
+
+        <div class="card-body p-4">
+
+            <p class="text-muted mb-4" style="font-size: 0.875rem; line-height: 1.5;">
+                Bitte melden Sie sich mit Ihren Zugangsdaten an, um auf das Sender-Dashboard zuzugreifen.
+            </p>
+
+            @if (LoginResult == SenderLoginResult.InvalidCredentials) {
+                <div class="alert alert-danger d-flex align-items-start gap-2 py-2" role="alert">
+                    <svg xmlns="http://www.w3.org/2000/svg" width="18" height="18" fill="currentColor" class="flex-shrink-0 mt-1" viewBox="0 0 16 16">
+                        <path d="M16 8A8 8 0 1 1 0 8a8 8 0 0 1 16 0zM5.354 4.646a.5.5 0 1 0-.708.708L7.293 8l-2.647 2.646a.5.5 0 0 0 .708.708L8 8.707l2.646 2.647a.5.5 0 0 0 .708-.708L8.707 8l2.647-2.646a.5.5 0 0 0-.708-.708L8 7.293 5.354 4.646z"/>
+                    </svg>
+                    <div>
+                        <strong>Ungültige Anmeldedaten.</strong><br />
+                        <span style="font-size:0.85rem;">Benutzername oder Passwort ist falsch. Bitte versuchen Sie es erneut.</span>
+                    </div>
+                </div>
+            } else if (LoginResult == SenderLoginResult.Error) {
+                <div class="alert alert-secondary d-flex align-items-start gap-2 py-2" role="alert">
+                    <svg xmlns="http://www.w3.org/2000/svg" width="18" height="18" fill="currentColor" class="flex-shrink-0 mt-1" viewBox="0 0 16 16">
+                        <path d="M8 15A7 7 0 1 1 8 1a7 7 0 0 1 0 14zm0 1A8 8 0 1 0 8 0a8 8 0 0 0 0 16z"/>
+                        <path d="M7.002 11a1 1 0 1 1 2 0 1 1 0 0 1-2 0zM7.1 4.995a.905.905 0 1 1 1.8 0l-.35 3.507a.552.552 0 0 1-1.1 0L7.1 4.995z"/>
+                    </svg>
+                    <div>
+                        <strong>Serverfehler.</strong><br />
+                        <span style="font-size:0.85rem;">Ein unerwarteter Fehler ist aufgetreten. Bitte versuchen Sie es später erneut.</span>
+                    </div>
+                </div>
+            }
+
+            <div class="mb-3">
+                <label class="form-label fw-medium" for="login-username">
+                    Benutzername
+                    <span class="text-danger ms-1">*</span>
+                </label>
+                <div class="input-group">
+                    <span class="input-group-text bg-light">
+                        <svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" fill="#6c757d" viewBox="0 0 16 16">
+                            <path d="M3 14s-1 0-1-1 1-4 6-4 6 3 6 4-1 1-1 1H3Zm5-6a3 3 0 1 0 0-6 3 3 0 0 0 0 6Z"/>
+                        </svg>
+                    </span>
+                    <input id="login-username"
+                           type="text"
+                           class="form-control @(LoginResult == SenderLoginResult.InvalidCredentials ? "is-invalid" : null)"
+                           placeholder="Benutzername eingeben"
+                           @bind="Username"
+                           @bind:event="oninput"
+                           @onkeydown="OnKeyDownAsync"
+                           disabled="@IsLoading"
+                           autocomplete="username" />
+                </div>
+            </div>
+
+            <div class="mb-4">
+                <label class="form-label fw-medium" for="login-password">
+                    Passwort
+                    <span class="text-danger ms-1">*</span>
+                </label>
+                <div class="input-group">
+                    <span class="input-group-text bg-light border-end-0">
+                        <svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" fill="#6c757d" viewBox="0 0 16 16">
+                            <path d="M8 1a2 2 0 0 1 2 2v4H6V3a2 2 0 0 1 2-2zm3 6V3a3 3 0 0 0-6 0v4a2 2 0 0 0-2 2v5a2 2 0 0 0 2 2h6a2 2 0 0 0 2-2V9a2 2 0 0 0-2-2z"/>
+                        </svg>
+                    </span>
+                    <input id="login-password"
+                           type="@(ShowPassword ? "text" : "password")"
+                           class="form-control border-start-0 border-end-0 @(LoginResult == SenderLoginResult.InvalidCredentials ? "is-invalid" : null)"
+                           placeholder="Passwort eingeben"
+                           @bind="Password"
+                           @bind:event="oninput"
+                           @onkeydown="OnKeyDownAsync"
+                           disabled="@IsLoading"
+                           autocomplete="current-password" />
+                    <button type="button"
+                            class="btn btn-outline-secondary border-start-0"
+                            style="border-left: none;"
+                            tabindex="-1"
+                            @onclick="() => ShowPassword = !ShowPassword">
+                        @if (ShowPassword) {
+                            <svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" fill="currentColor" viewBox="0 0 16 16">
+                                <path d="M13.359 11.238C15.06 9.72 16 8 16 8s-3-5.5-8-5.5a7.028 7.028 0 0 0-2.79.588l.77.771A5.944 5.944 0 0 1 8 3.5c2.12 0 3.879 1.168 5.168 2.457A13.134 13.134 0 0 1 14.828 8c-.058.087-.122.183-.195.288-.335.48-.83 1.12-1.465 1.755-.165.165-.337.328-.517.486l.708.709z"/>
+                                <path d="M11.297 9.176a3.5 3.5 0 0 0-4.474-4.474l.823.823a2.5 2.5 0 0 1 2.829 2.829l.822.822zm-2.943 1.299.822.822a3.5 3.5 0 0 1-4.474-4.474l.823.823a2.5 2.5 0 0 0 2.829 2.829z"/>
+                                <path d="M3.35 5.47c-.18.16-.353.322-.518.487A13.134 13.134 0 0 0 1.172 8l.195.288c.335.48.83 1.12 1.465 1.755C4.121 11.332 5.881 12.5 8 12.5c.716 0 1.39-.133 2.02-.36l.77.772A7.029 7.029 0 0 1 8 13.5C3 13.5 0 8 0 8s.939-1.721 2.641-3.238l.708.709z"/>
+                                <path fill-rule="evenodd" d="M13.646 14.354l-12-12 .708-.708 12 12-.708.708z"/>
+                            </svg>
+                        } else {
+                            <svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" fill="currentColor" viewBox="0 0 16 16">
+                                <path d="M16 8s-3-5.5-8-5.5S0 8 0 8s3 5.5 8 5.5S16 8 16 8zM1.173 8a13.133 13.133 0 0 1 1.66-2.043C4.12 4.668 5.88 3.5 8 3.5c2.12 0 3.879 1.168 5.168 2.457A13.133 13.133 0 0 1 14.828 8c-.058.087-.122.183-.195.288-.335.48-.83 1.12-1.465 1.755C11.879 11.332 10.119 12.5 8 12.5c-2.12 0-3.879-1.168-5.168-2.457A13.134 13.134 0 0 1 1.172 8z"/>
+                                <path d="M8 5.5a2.5 2.5 0 1 0 0 5 2.5 2.5 0 0 0 0-5zM4.5 8a3.5 3.5 0 1 1 7 0 3.5 3.5 0 0 1-7 0z"/>
+                            </svg>
+                        }
+                    </button>
+                </div>
+            </div>
+
+            <button class="btn btn-primary w-100 py-2 fw-medium"
+                    style="background: linear-gradient(135deg, #2c3e50 0%, #3498db 100%); border: none;"
+                    @onclick="SubmitAsync"
+                    disabled="@(IsLoading || string.IsNullOrWhiteSpace(Username) || string.IsNullOrWhiteSpace(Password))">
+                @if (IsLoading) {
+                    <span class="spinner-border spinner-border-sm me-2" role="status" aria-hidden="true"></span>
+                    <span>Anmelden …</span>
+                } else {
+                    <svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" fill="currentColor" class="me-2" viewBox="0 0 16 16">
+                        <path fill-rule="evenodd" d="M10 3.5a.5.5 0 0 0-.5-.5h-8a.5.5 0 0 0-.5.5v9a.5.5 0 0 0 .5.5h8a.5.5 0 0 0 .5-.5v-2a.5.5 0 0 1 1 0v2A1.5 1.5 0 0 1 9.5 14h-8A1.5 1.5 0 0 1 0 12.5v-9A1.5 1.5 0 0 1 1.5 2h8A1.5 1.5 0 0 1 11 3.5v2a.5.5 0 0 1-1 0v-2z"/>
+                        <path fill-rule="evenodd" d="M4.146 8.354a.5.5 0 0 1 0-.708l3-3a.5.5 0 1 1 .708.708L5.707 7.5H14.5a.5.5 0 0 1 0 1H5.707l2.147 2.146a.5.5 0 0 1-.708.708l-3-3z"/>
+                    </svg>
+                    <span>Anmelden</span>
+                }
+            </button>
+
+        </div>
+
+        <div class="card-footer text-center text-muted py-3 border-0 bg-transparent" style="font-size: 0.78rem;">
+            Bei Problemen wenden Sie sich bitte an den Administrator.
+        </div>
+
+    </div>
+</div>
+
+@code {
+    string Username = string.Empty;
+    string Password = string.Empty;
+    bool ShowPassword;
+    bool IsLoading;
+    SenderLoginResult? LoginResult;
+
+    async Task OnKeyDownAsync(Microsoft.AspNetCore.Components.Web.KeyboardEventArgs e) {
+        if (e.Key == "Enter")
+            await SubmitAsync();
+    }
+
+    async Task SubmitAsync() {
+        if (string.IsNullOrWhiteSpace(Username) || string.IsNullOrWhiteSpace(Password) || IsLoading) return;
+
+        IsLoading = true;
+        LoginResult = null;
+        await InvokeAsync(StateHasChanged);
+
+        var result = await AuthService.LoginSenderAsync(Username.Trim(), Password.Trim());
+
+        if (result == SenderLoginResult.Success) {
+            Navigation.NavigateTo("/", forceLoad: true);
+            return;
+        }
+
+        LoginResult = result;
+        IsLoading = false;
+        await InvokeAsync(StateHasChanged);
+    }
+}
+