feat: explain when signer has no visible signature field

[nva138]

Resolves: #8325

📝 Summary

Shows an NcNoteCard in the signing sidebar when the current signer has no visible signature field, so the page does not look incomplete.

Reuses hasVisibleElementsForCurrentUser(). I could not reuse needCreateSignature, it also returns false when the instance disallows creating signatures or when the user already has one.

The card is a sibling of the existing v-if chain, not part of it, so "Sign document" stays available.

🧪 How to test

1. Upload a PDF in LibreSign
2. Add yourself as signer, but do not place a visible signature element
3. Request signatures and open the document to sign
4. The sidebar shows the note card above the "Sign document" button
5. Signing still works normally

npx vitest run src/tests/views/SignPDF/Sign.spec.ts -t "no visible signature notice"
npx playwright test sign-herself-with-click-to-sign

🎨 UI / Front-end changes

• Informational note card when the signer has no visible signature field
• Screenshots before/after

🏚️ Before	🏡 After

• Tested in multiple browsers (Chrome, Firefox, Safari)
• Components, Unit (with vitest) and/or e2e (with Playwright) tests added
• Accessibility verified (contrast, keyboard navigation, screen reader friendly)
• Design review approved
• Documentation updated (if applicable)

Tests: component tests for both cases in Sign.spec.ts, and extended sign-herself-with-click-to-sign which already signs without a visible field.

One thing worth your review: the card also shows while a non-retriable certificate error blocks signing, so "Sign.vue - API error handling" now expects 2 note cards instead of 1. The issue describes the condition without exceptions so I kept it, but tell me if you want it hidden there.

✅ Checklist

• I have read and followed the contribution guide.
• Conventional commits with DCO sign-off
• npm test, npm run lint and npm run ts:check pass

🤖 AI (if applicable)

• The content of this PR was partially generated using AI

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Files with missing lines	Coverage Δ
src/views/SignPDF/_partials/Sign.vue	59.93% <100.00%> (+8.14%)	⬆️

... and 220 files with indirect coverage changes

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