Add envelope editor with PDF upload and signature tools

Introduced a new Blazor page `EnvelopeSenderEditorPage.razor` for editing envelopes with an interactive interface. Integrated `DxPdfViewer` for rendering PDFs and added functionality for uploading, viewing, and interacting with PDF files.

Key features:
- Action bar with buttons for uploading PDFs, toggling signature placement mode, clearing fields, and saving.
- Placement mode for adding signature fields via an overlay, with visual placeholders.
- JavaScript interop (`envelope-editor.js`) for precise click coordinate mapping.
- Error handling for unsupported file types and size limits (50 MB).
- Logging for debugging key actions like PDF uploads and field placements.

Defined constants for accurate signature field dimensions and scaling. Added models (`SignatureFieldDraft`, `OverlayCoords`) to manage state and interactions.

EnvelopeGenerator.Server/EnvelopeGenerator.Server/wwwroot/js/envelope-editor.js

@@ -0,0 +1,21 @@
+window.envelopeEditor = {
+    /**
+     * Returns click coordinates relative to the overlay element.
+     * @param {string} overlayId - The id of the overlay div
+     * @param {number} clientX   - MouseEventArgs.ClientX from Blazor
+     * @param {number} clientY   - MouseEventArgs.ClientY from Blazor
+     * @returns {{ relX, relY, containerW, containerH }}
+     */
+    getClickCoords: function (overlayId, clientX, clientY) {
+        const el = document.getElementById(overlayId);
+        if (!el) return null;
+
+        const rect = el.getBoundingClientRect();
+        return {
+            relX: clientX - rect.left,
+            relY: clientY - rect.top,
+            containerW: rect.width,
+            containerH: rect.height
+        };
+    }
+};