Add PDF editing and envelope management features

Introduced a new `SenderEditorPage` with a modern UI for managing envelopes, uploading PDFs, and placing signature fields. Integrated `DevExpress.Blazor.PdfViewer` for PDF rendering and interaction.

Key features include:
- PDF upload, rendering, and validation.
- Signature field placement with normalized coordinates.
- Receiver management with validation and email suggestions.
- Envelope metadata handling (title, message).
- Save as draft and send functionality with validation.
- Edit mode for existing envelopes.
- Improved UX with dynamic elements and color-coded receivers.
- Localization support for multi-language UI.

Updated `_Imports.razor` to include necessary namespaces for DevExpress components and the new `PdfEditor` namespace.

EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Components/PdfEditor/SenderPdfImageEditor.razor

@@ -0,0 +1,31 @@
+@using DevExpress.Blazor.PdfViewer
+
+<div class="pdf-editor-wrapper"
+     style="cursor: @(IsPlacementMode ? "crosshair" : "default");"
+     @onclick="HandleClickAsync">
+    @if (PdfBytes is { Length: > 0 })
+    {
+        <DxPdfViewer DocumentContent="@PdfBytes"
+                     CssClass="@ViewerCssClass" />
+    }
+</div>
+
+@code {
+    [Parameter]
+    public byte[]? PdfBytes { get; set; }
+
+    [Parameter]
+    public bool IsPlacementMode { get; set; }
+
+    [Parameter]
+    public string ViewerCssClass { get; set; } = "sender-editor-pdf-viewer";
+
+    [Parameter]
+    public EventCallback<MouseEventArgs> EditorClick { get; set; }
+
+    private async Task HandleClickAsync(MouseEventArgs args)
+    {
+        if (EditorClick.HasDelegate)
+            await EditorClick.InvokeAsync(args);
+    }
+}