Add signed result page and update navigation flow

Introduced a new `/envelope/signed-result` page to confirm
successful document signing with localized messages and
a visually appealing UI. Updated `ReceiverSignedPage.razor`
to redirect users to the new page after signing.

Enhanced styles in `app.css` for the signed result page,
including responsive layout, theme-specific designs, and
typography improvements. Added support for localization
using `IStringLocalizer` with fallback strings in German.

Improves user experience by providing clear feedback and
a polished confirmation page after signing.

EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Pages/EnvelopeSignedResultPage.razor

@@ -0,0 +1,29 @@
+@page "/envelope/signed-result"
+@rendermode InteractiveWebAssembly
+@using EnvelopeGenerator.Application.Resources
+@using Microsoft.Extensions.Localization
+@inject IStringLocalizer<Resource> UiLocalizer
+
+<div class="receiver-signed-result-page">
+    <div class="receiver-signed-result-card">
+        <div class="receiver-signed-result-icon" aria-hidden="true">
+            <svg xmlns="http://www.w3.org/2000/svg" width="72" height="72" fill="currentColor" viewBox="0 0 16 16">
+                <path d="M2.5 8a5.5 5.5 0 0 1 8.25-4.764.5.5 0 0 0 .5-.866A6.5 6.5 0 1 0 14.5 8a.5.5 0 0 0-1 0 5.5 5.5 0 1 1-11 0z" />
+                <path d="M15.354 3.354a.5.5 0 0 0-.708-.708L8 9.293 5.354 6.646a.5.5 0 1 0-.708.708l3 3a.5.5 0 0 0 .708 0l7-7z" />
+            </svg>
+        </div>
+
+        <h1 class="receiver-signed-result-title">@L("DocumentSuccessfullySigned", "Dokument erfolgreich unterzeichnet")</h1>
+        <p class="receiver-signed-result-message">
+            @L("DocumentSignedConfirmationMessage", "Vielen Dank. Ihre Unterschrift wurde erfolgreich gespeichert und an die beteiligten Parteien übermittelt.")
+        </p>
+    </div>
+</div>
+
+@code {
+    string L(string key, string fallback)
+    {
+        var value = UiLocalizer[key];
+        return value.ResourceNotFound ? fallback : value.Value;
+    }
+}

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Components/Pages/ReceiverSignedPage.razor

@@ -174,7 +174,7 @@
         _submitConfirmVisible = false;
         await InvokeAsync(StateHasChanged);
         await AuthService.LogoutEnvelopeReceiverAsync(EnvelopeKey!);
-        Navigation.NavigateTo("/", forceLoad: true);
+        Navigation.NavigateTo("/envelope/signed-result", forceLoad: true);
     }
 
     protected override async Task OnInitializedAsync()

EnvelopeGenerator.Server/EnvelopeGenerator.Server/wwwroot/css/app.css

@@ -345,6 +345,83 @@ article {
     font-weight: 500;
 }
 
+/* Signed result page */
+.receiver-signed-result-page {
+    min-height: calc(100vh - 36px);
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    padding: 2rem 1rem;
+    background: linear-gradient(160deg, #1e2e3e 0%, #2c3e50 40%, #3498db 100%);
+}
+
+.receiver-signed-result-card {
+    width: min(680px, 100%);
+    background: rgba(255, 255, 255, 0.96);
+    border: 1px solid rgba(255, 255, 255, 0.7);
+    border-radius: 18px;
+    box-shadow: 0 18px 44px rgba(15, 23, 42, 0.24);
+    padding: 2rem 1.5rem;
+    text-align: center;
+}
+
+.receiver-signed-result-icon {
+    display: inline-flex;
+    align-items: center;
+    justify-content: center;
+    width: 92px;
+    height: 92px;
+    border-radius: 50%;
+    color: #0f8f58;
+    background: linear-gradient(135deg, #dcfce7 0%, #bbf7d0 100%);
+    border: 1px solid #86efac;
+    margin-bottom: 1rem;
+}
+
+.receiver-signed-result-title {
+    margin: 0;
+    color: #1f2937;
+    font-size: 1.7rem;
+    font-weight: 700;
+    letter-spacing: -0.02em;
+}
+
+.receiver-signed-result-message {
+    margin: 0.85rem auto 0;
+    max-width: 58ch;
+    color: #4b5563;
+    font-size: 1rem;
+    line-height: 1.62;
+}
+
+[data-sf-theme="purple"] .receiver-signed-result-page {
+    background: linear-gradient(160deg, #1a0533 0%, #3b0764 45%, #6b21a8 100%);
+}
+
+[data-sf-theme="blazing-dark"] .receiver-signed-result-page {
+    background: linear-gradient(160deg, #0a0f1e 0%, #111827 50%, #1e1b4b 100%);
+}
+
+[data-sf-theme="blazing-dark"] .receiver-signed-result-card {
+    background: rgba(15, 20, 40, 0.94);
+    border-color: rgba(129, 140, 248, 0.28);
+    box-shadow: 0 20px 46px rgba(0, 0, 0, 0.45);
+}
+
+[data-sf-theme="blazing-dark"] .receiver-signed-result-title {
+    color: rgba(255, 255, 255, 0.92);
+}
+
+[data-sf-theme="blazing-dark"] .receiver-signed-result-message {
+    color: rgba(255, 255, 255, 0.68);
+}
+
+[data-sf-theme="blazing-dark"] .receiver-signed-result-icon {
+    color: #86efac;
+    background: rgba(16, 185, 129, 0.16);
+    border-color: rgba(110, 231, 183, 0.35);
+}
+
 /* ── Logos ───────────────────────────────────────────────────────────────── */
 /* Heights are controlled by inline style="height: Xpx" from appsettings.json */