Add signature input and PDF export functionality

Introduced a signature input feature in `ReportViewer.razor` to allow users to draw a signature on a canvas and embed it into reports before exporting as a signed PDF.

- Added a canvas (`receiver-signature-pad`) and buttons for clearing, applying, and exporting signatures.
- Injected `IJSRuntime` for JavaScript interop and added methods for signature handling (`ClearSignatureAsync`, `ApplySignatureAsync`, `ExportSignedPdfAsync`).
- Embedded the signature as an image and label in the report's bottom margin.
- Added `receiver-signature.js` to manage the signature pad, including drawing, clearing, and exporting the signature as a Base64 image.
- Updated `index.html` to include the new JavaScript file.
- Displayed validation messages for missing or invalid signatures.

EnvelopeGenerator.ReceiverUI/wwwroot/js/receiver-signature.js

@@ -0,0 +1,86 @@
+window.receiverSignature = (() => {
+    const pads = new Map();
+
+    function getPosition(canvas, event) {
+        const rect = canvas.getBoundingClientRect();
+        const source = event.touches && event.touches.length ? event.touches[0] : event;
+        return {
+            x: (source.clientX - rect.left) * (canvas.width / rect.width),
+            y: (source.clientY - rect.top) * (canvas.height / rect.height)
+        };
+    }
+
+    function initialize(canvasId) {
+        const canvas = document.getElementById(canvasId);
+        if (!canvas || pads.has(canvasId))
+            return;
+
+        const context = canvas.getContext('2d');
+        context.lineWidth = 2.5;
+        context.lineCap = 'round';
+        context.lineJoin = 'round';
+        context.strokeStyle = '#111';
+
+        const state = { drawing: false, hasSignature: false };
+        pads.set(canvasId, state);
+
+        const start = event => {
+            event.preventDefault();
+            const pos = getPosition(canvas, event);
+            state.drawing = true;
+            context.beginPath();
+            context.moveTo(pos.x, pos.y);
+        };
+
+        const move = event => {
+            if (!state.drawing)
+                return;
+
+            event.preventDefault();
+            const pos = getPosition(canvas, event);
+            context.lineTo(pos.x, pos.y);
+            context.stroke();
+            state.hasSignature = true;
+        };
+
+        const end = event => {
+            if (!state.drawing)
+                return;
+
+            event.preventDefault();
+            state.drawing = false;
+        };
+
+        canvas.addEventListener('mousedown', start);
+        canvas.addEventListener('mousemove', move);
+        window.addEventListener('mouseup', end);
+        canvas.addEventListener('touchstart', start, { passive: false });
+        canvas.addEventListener('touchmove', move, { passive: false });
+        canvas.addEventListener('touchend', end, { passive: false });
+    }
+
+    function clear(canvasId) {
+        const canvas = document.getElementById(canvasId);
+        const state = pads.get(canvasId);
+        if (!canvas || !state)
+            return;
+
+        canvas.getContext('2d').clearRect(0, 0, canvas.width, canvas.height);
+        state.hasSignature = false;
+    }
+
+    function getDataUrl(canvasId) {
+        const canvas = document.getElementById(canvasId);
+        const state = pads.get(canvasId);
+        if (!canvas || !state || !state.hasSignature)
+            return null;
+
+        return canvas.toDataURL('image/png');
+    }
+
+    return {
+        initialize,
+        clear,
+        getDataUrl
+    };
+})();