When viewing PDF, show a "digitally signed" icon on the top bar

Is your feature request related to a problem? Please describe.
When viewing PDFs, it might be hard to see that it was digitally signed.

Describe the solution you'd like

Create/add a "universal" icon next to the three dots on the top navbar when the opened PDF has any digital signature, instead

Hovering would show a "This file contains digital signatures, click for more details."
On click, the sidebar could open and show the same details as it would on the "LibreSign" tab when we select the file in Files. See https://github.com/LibreSign/libresign/issues/4164

to avoid people who feel like this would create clutter, add an admin option to show/hide this by default.

Describe alternatives you've considered

Additional context
I saw an issue about the nextcloud pdf viewer, not sure what happened with that, but I cannot open the sidebar from those three dots. 🤷

• Related: Icons part of this reply https://github.com/LibreSign/libresign/issues/4160#issuecomment-2548723509

Start by reviewing the PDF viewer's top navbar and the existing LibreSign tab/sidebar behavior, then read related issues 4160 and 4164 for the icon and signature details. Done means signed PDFs show the requested icon with hover and click behavior, and an admin option controls its default visibility.