fix(validation): open signed PDF by UUID instead of the owner's nodeIdFixes #9103 - #9104

Resolves: #9103

📝 Summary

On the validation page, "Open file" / "View PDF" passed the file owner's original nodeId to OCA.Viewer for logged-in users. Viewer handlers that resolve fileid in the current user's filesystem (Euro-Office, ONLYOFFICE) then:

• showed the owner the unsigned original instead of the signed PDF;
• failed with {"error":"File not found"} for signers and other logged-in users.

This PR makes the validation page always open the signed PDF by UUID (/apps/libresign/p/pdf/<uuid>), as it already does for anonymous visitors. openDocument now only uses OCA.Viewer when a nodeId is explicitly given, so other callers (Files list, sidebar, ID docs, settings) are unchanged.

The change is frontend only. The backend route already resolves the storage owner from the UUID and serves the signed file.

🧪 How to test

1. Install Euro-Office (or ONLYOFFICE) and enable PDF as a default format in its admin settings.
2. As user X, request signatures from Nextcloud users Y and Z without sharing the file. Have both sign.
3. Open /apps/libresign/validation/<uuid> and click "Open file":
   • as X: the signed PDF opens (before: unsigned original);
   • as Y, and as an unrelated logged-in user W: the signed PDF opens (before: "File not found");
   • logged out: the signed PDF still opens.
4. Optional: repeat with PDF not set as a Euro-Office default. Behaviour is the same.
5. Optional: for an envelope, check that "View PDF" on each file opens that file's signed PDF.

Validation evidence

• Focused regression/acceptance test
• Relevant lint/type/static-analysis checks
• Broader suite required by the changed surface
• CI reviewed

🎨 UI / Front-end changes

• The validation page opens the signed PDF by UUID for every visitor.
• Logged-in users now get the PDF in a new tab instead of the Viewer overlay, as anonymous users already do on main. The Viewer picks its handler by MIME type, and some handlers need a node the user can access, so it can't be used safely here.
• In EnvelopeValidation, "View PDF" is shown when a file has a uuid, instead of requiring nodeId.

🚧 Tasks

• Manual check with Euro-Office as the default PDF viewer (owner, signer, unrelated user, anonymous)

✅ Checklist

• I have read and followed the contribution guide.
• The diff does not contain unrelated refactors or generated changes.
• Commits follow Conventional Commits where applicable.
• All commits include the required DCO sign-off.

🤖 AI (if applicable)

• Only the tests suite was generated using AI;