Localize "envelope expired" page and add resource keys

Added "DocumentSharingPeriodExpired" and "Expired" keys to de-DE, en-US, and fr-FR resource files. Updated EnvelopeExpired.cshtml to use localized strings for the title, heading, and message, enabling full internationalization of the expired envelope page.

EnvelopeGenerator.Web/Views/Envelope/EnvelopeExpired.cshtml

@@ -1,5 +1,5 @@
 @{
-    ViewData["Title"] = "Abgelaufen";
+    ViewData["Title"] = _localizer["Expired"];
 }
 <div class="page container p-5">
     <header class="text-center">
@@ -15,9 +15,9 @@
                 <path fill="#263238" d="M26.4,39.9c0-0.2,0-0.4,0.1-0.6s0.2-0.3,0.3-0.5s0.3-0.2,0.5-0.3s0.4-0.1,0.6-0.1s0.5,0,0.7,0.1 s0.4,0.2,0.5,0.3s0.2,0.3,0.3,0.5s0.1,0.4,0.1,0.6s0,0.4-0.1,0.6s-0.2,0.3-0.3,0.5s-0.3,0.2-0.5,0.3s-0.4,0.1-0.7,0.1 s-0.5,0-0.6-0.1s-0.4-0.2-0.5-0.3s-0.2-0.3-0.3-0.5S26.4,40.1,26.4,39.9z M29.2,36.8h-2.3L26.5,27h3L29.2,36.8z" />
             </svg>
         </div>
-        <h1>Abgelaufen!</h1>
+        <h1>@_localizer["Expired"]</h1>
     </header>
     <section class="text-center">
-        <p>Der Zeitraum für die gemeinsame Nutzung von Dokumenten ist abgelaufen.</p>
+        <p>@_localizer["DocumentSharingPeriodExpired"]</p>
     </section>
 </div>