Refactor: Consolidate Shared Preferences Keys

[davidetarsi]

This pull request refactors the management of shared preferences by grouping all preference keys in a single location for better maintainability and consistency.

Changes made:

• Grouped all shared preferences keys, including both normal keys and enabled keys, in the const.dart file for centralized management.
• Did not update the hardcoded strings in sp.getString() calls to use the newly defined spKey constants to avoid potential regressions.

Benefits:

• Improved code organization and maintainability by consolidating preference keys.
• Simplified future updates, reducing the risk of inconsistencies.

Notes for Reviewers:

• Updating the sp.getString() calls with the new constants is planned for a future update.
• Please verify that no existing functionality has been impacted by the changes to key declarations.