[feat] Implement robust localization infrastructure (l10n)

[iberi22]

Problem: The app lacks a proper localization system and has hardcoded Spanish strings. Goal: Setup flutter_localizations, l10n.yaml, and initial ARB files in lib/l10n for English and Spanish.

[google-labs-jules]

Jules is on it. When finished, you will see another comment and be able to review a PR.

[google-labs-jules]

Ready for a review! A PR has been created.