feat: migrate i18n system from custom fluent to fluent-templates
- Replace custom Fluent implementation with fluent-templates crate
- Fix FluentValue type conversions and remove DateTime variant support
- Update translation methods to return String directly instead of Option<String>
- Maintain backward compatibility for existing form validation calls
- Resolve all compilation errors and ensure tests pass
BREAKING CHANGE: i18n system now uses fluent-templates instead of custom implementation