Replace magic strings with enums in sorting logic

[arukompas]

Summary

• Create new SortingMethod enum alongside deprecated FolderSortingMethod for backward compatibility
• Replace all magic string direction values ('asc'/'desc') with SortingOrder enum constants
• Add validateDirection() method to both FilesController and FoldersController to gracefully handle invalid direction values, defaulting to descending order
• Add comprehensive test coverage for invalid sorting inputs in InvalidSortingValuesTest

Test Plan

• All 273 existing tests pass
• New validation tests verify invalid direction values default to descending
• Verify alphabetical and time-based sorting still work correctly
• Code formatting passes with Laravel Pint

Changes

• ✨ Create new SortingMethod enum
• 📝 Mark FolderSortingMethod as deprecated
• ♻️ Replace magic strings with enum constants
• 🛡️ Add input validation for sorting direction
• 🧪 Add comprehensive validation tests