feat: Inline preview for spreadsheet attachments (.xlsx / .csv)

Feature request: inline preview for spreadsheet attachments Is your feature request related to a problem? When a user attaches a .xlsx (or .csv) file to a document it renders as a plain attachment widget (file icon + name + size). There is no way to view the spreadsheet content without downloading the file and opening it in a separate app. Only PDF attachments currently support an inline preview (shared/editor/nodes/Attachment.tsx renders a PdfViewer when preview is set), so spreadsheet support is a natural extension of that path. I searched existing issues (excel, xlsx, spreadsheet, csv preview) and could not find a duplicate feature request. Describe the solution you'd like • Uploading/dragging an .xlsx (and ideally .csv) file should offer an inline preview of the sheet content, similar to the existing PDF preview. • Rendering should happen client-side (e.g. SheetJS / exceljs) so it works in fully offline / intranet deployments and does not depend on an ex…