fix(email): replace html2pdf with Print / save PDF and harden EML export

SAN-157 removed the html2pdf client library. The PDF export code added
in the original PR commit referenced the deleted library and would fail
at runtime. Replace the html2pdf-based _downloadSelectedEmailPdf with a
browser Print / save PDF workflow using a sandboxed iframe and
window.print(), matching the established pattern in document.js and
compare/index.js.

Changes:
- Replace _loadEmailHtml2Pdf and _downloadSelectedEmailPdf with
  _printSelectedEmails using a sandboxed iframe + contentWindow.print()
- Update menu label from 'PDF' to 'Print / save PDF'
- Add 512 MiB total size cap to /messages-download-bulk to prevent
  unbounded memory growth when exporting many large messages
- Update test assertions to verify the browser print pattern and
  confirm html2pdf is not referenced
- Add regression tests for size cap, selection count validation,
  non-numeric UID rejection, and partial batch failure accounting

static/js/emailLibrary/menus.js

@@ -619,7 +619,7 @@ export function _showBulkActionsMenu(anchor) {
         dropdown.appendChild(divider);
         const options = [
           { label: 'Email file (.eml)', icon: _emailIco, action: _exportSelectedEmails },
-          { label: 'PDF', icon: _pdfIco, action: _exportSelectedEmailPdf },
+          { label: 'Print / save PDF', icon: _pdfIco, action: _exportSelectedEmailPdf },
         ];
         for (const option of options) {
           const child = document.createElement('div');