Add robust input validation system and fix test suite
- Add comprehensive validation package with email (RFC 5322), password strength, and IANA timezone validation
- Implement strict password requirements: min 8 chars, uppercase, lowercase, digit, special character
- Integrate validation into RegisterUserHandler with complete test coverage (59 validation tests + 25 handler tests)
- Fix pre-existing test failures:
- Remove tests for non-existent HabitEntry.DeletedAt and Delete() methods
- Replace deprecated HabitTypeQuantity with HabitTypeValue
- Add missing FindByHabitIDAndDateRange mock implementation
- Remove hardcoded localhost:8080 from Swagger config for self-hosted flexibility
david
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