Enhance signature navigation with current index display

Added functionality to display the current signature index in the
signature counter UI in `EnvelopeViewer.razor`, including a new
state variable `_currentSignatureIndex` to track the currently
viewed signature. Updated the logic to fetch and set this value
from the `SignatureNavState` object provided by the JavaScript
runtime.

Modified `pdf-viewer.js` to calculate the current signature index
based on the last viewed signature ID (`_lastViewedSignatureId`)
and return it as part of the `SignatureNavState`. This replaces
the previously hardcoded `signed` value.

These changes improve the user experience by providing a clear
indication of the currently viewed signature in the navigation UI.

EnvelopeGenerator.ReceiverUI/wwwroot/js/pdf-viewer.js

@@ -505,11 +505,18 @@ window.pdfViewer = {
         const signed = this.appliedSignatures.length;       // ?mzalananlar
         const unsigned = total - signed;                    // Hesaplanan: ?mzalanmayanlar
         
+        // Mevcut görüntülenen imzanın sıra numarasını bul
+        let currentIndex = 0;
+        if (this._lastViewedSignatureId) {
+            const index = this._allSignatures.findIndex(s => s.id === this._lastViewedSignatureId);
+            currentIndex = index !== -1 ? index + 1 : 0;  // 1-based index (kullanıcıya göstermek için)
+        }
+                
         return {
             total: total,
             signed: signed,
             unsigned: unsigned,
-            currentIndex: signed,
+            currentIndex: currentIndex,  // Şu an hangi imzada (1-5 arası)
             canGoPrev: total > 0,           // Her zaman aktif (e?er imza varsa)
             canGoNext: total > 0            // Her zaman aktif (e?er imza varsa)
         };
@@ -1017,3 +1024,4 @@ window.pdfViewer = {
 
 
 
+