Add Portuguese UI and improve PDF translation robustness

[jacksonbarreto]

Summary

This PR adds Portuguese UI support and improves PDF translation robustness.

Changes included:

• Add Portuguese translations to the UI.
• Add Portuguese to the interface language selector.
• Set default translation direction to English → Portuguese.
• Translate fallback author labels to Portuguese.
• Add retries and fallback behavior for third-party translation APIs, avoiding blank PDF blocks when transient API errors occur.
• Normalize common PDF ligatures before translation.
• Prevent oversized decorative text, such as mastheads, logos, and drop caps, from being merged with normal text.
• Preserve oversized decorative blocks during PDF redrawing to reduce layout destruction.

Notes

The changes were tested locally with text-based academic PDFs and OpenAI-compatible third-party translation endpoints.

The retry fallback returns the original text if translation fails after retries, avoiding empty inserted blocks.