Convert storage from legacy preferences (about:config) to local storage (API compatible)

[RealRaven2000]

Migrate settings from legacy preferences to extension local storage

Move all quickFilters settings from Thunderbird's legacy preference system (about:config) to WebExtension local storage.

Benefits

• Settings are isolated from other extensions.
• Uninstalling quickFilters removes all stored settings.
• Reduces clutter in Thunderbird's global preference database.
• Allows structured storage of complex settings objects.
• Supports export/import of settings and license information via JSON backup files.

Requirements

• Add "storage" permission to the manifest.
• Implement automatic one-time migration of existing preferences.
• Verify license handling during migration.
• Add backup / restore functionality.
• Remove obsolete preference code after migration is complete.