fix: move hardcoded strings to translations

[Sembauke]

Checklist:

• I have read and followed the contribution guidelines.
• I have read and followed the how to open a pull request guide.
• My pull request targets the main branch of the repo.
• I have tested these changes locally on my machine.

This moves remaining hardcoded user-facing mobile app strings into the ARB localization files and updates call sites to read them through the generated localization API.

• Adds missing localization keys across English, Spanish, and Portuguese ARB files.
• Regenerates Flutter localization classes.
• Localizes newer learn, quiz, daily challenge, forum help, login error, quick action, podcast, and news labels/messages.

Testing:

• flutter gen-l10n
• flutter analyze
• flutter test test/unit/challenge_utils_test.dart