feat(l10n): add TRANSLATORS hints for signing engine strings

[lfals]

Resolves: #973

📝 Summary

Adds 15 // TRANSLATORS comments for PHP signing-engine strings in:

• lib/Handler/SignEngine/Pkcs12Handler.php (signature validation labels and engine errors)
• lib/Handler/SignEngine/SignEngineHandler.php (certificate generation, PFX storage, signing password errors)

Hints give LibreSign context for translators (validation UI / signer certificate workflow) without changing English source strings. This is a 15-hint batch for #973.

Comments will appear in Transifex after the next extraction sync.

🧪 How to test

1. Open the two files above and confirm each changed $this->l10n->t(...) has a // TRANSLATORS comment on the line above.
2. Confirm the diff is comments only — no English source strings changed.
3. Optional: open LibreSign document validation for a signed PDF and confirm status labels still read the same in English.

⚙️ API / Back‑end changes

• Added PHP TRANSLATORS hints for signing-engine validation and certificate/password errors
• Unit and/or integration tests added – N/A: comment-only change; no behavior change
• Capabilities updated (if applicable)
• Documentation updated (if applicable)
• API documentation updated with composer openapi – N/A

✅ Checklist

• I have read and followed the contribution guide.
• Scoped to about 15 PHP translation hints as suggested in Implement translation hints in PHP #973
• No runtime behavior or source strings changed
• Conventional commit type uses an allowed prefix (feat)

🤖 AI (if applicable)

• The content of this PR was partially or fully generated using AI

Made with Cursor

[vitormattos]

/backport to stable34

[vitormattos]

/backport to stable33

[vitormattos]

/backport to stable32