Add authentication support with login/logout UI

- Introduced AuthService, IAuthApiClient, and AuthApiClient for managing authentication state and API calls (login, logout, session restore).
- Added Login.razor and LoginLayout.razor for the login page, including styling and logic.
- MainLayout.razor now checks authentication on load, restores sessions from sessionStorage, and redirects to /login if unauthenticated. Displays username and logout button when logged in.
- Implemented JS interop (authStorage) for persisting authentication info in sessionStorage.
- Registered AuthService, CookieContainer, and API clients in Program.cs to share cookies and support authentication.
- Updated AppSettings and appsettings files to support separate ApiBaseUrl and DataApiBaseUrl.
- Minor CSS improvements for username display in the top bar.

DbFirst.BlazorWebApp/Components/Pages/Login.razor

@@ -0,0 +1,106 @@
+@page "/login"
+@layout DbFirst.BlazorWebApp.Components.Layout.LoginLayout
+@rendermode InteractiveServer
+
+@inject IAuthApiClient AuthApiClient
+@inject AuthService AuthService
+@inject NavigationManager Navigation
+@inject IJSRuntime JS
+
+<PageTitle>Anmelden – DbFirst</PageTitle>
+
+<div class="login-container">
+    <div class="login-card">
+        <div class="login-header">
+            <span class="login-brand">DbFirst</span>
+            <p class="login-subtitle">Bitte melden Sie sich an</p>
+        </div>
+
+        <form @onsubmit="HandleLoginAsync" @onsubmit:preventDefault>
+            <div class="login-field">
+                <label class="login-label" for="username">Benutzername</label>
+                <input id="username"
+                       type="text"
+                       class="login-text-input"
+                       placeholder="Benutzername eingeben"
+                       autocomplete="username"
+                       @bind="_username"
+                       @bind:event="oninput" />
+            </div>
+
+            <div class="login-field">
+                <label class="login-label" for="password">Passwort</label>
+                <input id="password"
+                       type="password"
+                       class="login-text-input"
+                       placeholder="Passwort eingeben"
+                       autocomplete="current-password"
+                       @bind="_password"
+                       @bind:event="oninput" />
+            </div>
+
+            @if (!string.IsNullOrEmpty(_errorMessage))
+            {
+                <div class="alert alert-danger mt-2 mb-1" role="alert">
+                    @_errorMessage
+                </div>
+            }
+
+            <div class="login-actions">
+                <button type="submit"
+                        class="login-submit-btn"
+                        disabled="@_isLoading">
+                    @(_isLoading ? "Wird angemeldet…" : "Anmelden")
+                </button>
+            </div>
+        </form>
+    </div>
+</div>
+
+@code {
+    private string _username = string.Empty;
+    private string _password = string.Empty;
+    private string _errorMessage = string.Empty;
+    private bool _isLoading;
+
+    protected override void OnInitialized()
+    {
+        if (AuthService.IsAuthenticated)
+            Navigation.NavigateTo("/");
+    }
+
+    private async Task HandleLoginAsync()
+    {
+        if (_isLoading) return;
+        _errorMessage = string.Empty;
+
+        if (string.IsNullOrWhiteSpace(_username) || string.IsNullOrWhiteSpace(_password))
+        {
+            _errorMessage = "Bitte Benutzername und Passwort eingeben.";
+            return;
+        }
+
+        _isLoading = true;
+        try
+        {
+            var success = await AuthApiClient.LoginAsync(_username, _password);
+            if (success)
+            {
+                await JS.InvokeVoidAsync("authStorage.set", AuthService.UserName, AuthService.RawCookieHeader);
+                Navigation.NavigateTo("/");
+            }
+            else
+            {
+                _errorMessage = "Anmeldung fehlgeschlagen. Bitte Benutzerdaten prüfen.";
+            }
+        }
+        catch
+        {
+            _errorMessage = "Verbindungsfehler. Bitte später erneut versuchen.";
+        }
+        finally
+        {
+            _isLoading = false;
+        }
+    }
+}