Add signature panel and layout updates for ReportViewer

Enhanced `ReportViewer.razor` with a new layout structure:
- Added `receiver-page-layout` with `receiver-signature-panel` and `receiver-viewer-wrapper` for better organization.
- Introduced a button to export signed PDFs, conditionally enabled based on `SignatureApplied`.
- Added a `DxPopup` for capturing signatures with a "Close" button.

Updated `MainLayout.razor` to remove unnecessary padding from `<article>`.

Refined `app.css`:
- Defined styles for `receiver-page-layout`, `receiver-signature-panel`, and `receiver-viewer-wrapper` to improve layout flexibility.
- Adjusted `article` to use flexbox and ensure hidden overflow.

EnvelopeGenerator.sln

@@ -87,10 +87,6 @@ Global
 		{EC768913-6270-14F4-1DD3-69C87A659462}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{EC768913-6270-14F4-1DD3-69C87A659462}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{EC768913-6270-14F4-1DD3-69C87A659462}.Release|Any CPU.Build.0 = Release|Any CPU
-		{FB2D306B-1042-4A70-31ED-F991A1599371}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
-		{FB2D306B-1042-4A70-31ED-F991A1599371}.Debug|Any CPU.Build.0 = Debug|Any CPU
-		{FB2D306B-1042-4A70-31ED-F991A1599371}.Release|Any CPU.ActiveCfg = Release|Any CPU
-		{FB2D306B-1042-4A70-31ED-F991A1599371}.Release|Any CPU.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE
@@ -110,7 +106,6 @@ Global
 		{211619F5-AE25-4BA5-A552-BACAFE0632D3} = {9943209E-1744-4944-B1BA-4F87FC1A0EEB}
 		{224C4845-1CDE-22B7-F3A9-1FF9297F70E8} = {0CBC2432-A561-4440-89BC-671B66A24146}
 		{EC768913-6270-14F4-1DD3-69C87A659462} = {E3C758DC-914D-4B7E-8457-0813F1FDB0CB}
-		{FB2D306B-1042-4A70-31ED-F991A1599371} = {E3C758DC-914D-4B7E-8457-0813F1FDB0CB}
 	EndGlobalSection
 	GlobalSection(ExtensibilityGlobals) = postSolution
 		SolutionGuid = {73E60370-756D-45AD-A19A-C40A02DACCC7}