feat: add option to reset all settings to defaults #3355

Describe the feature

The settings page has no way to go back to the defaults. Once someone has changed the accent color, themes, toggles, search provider and so on, the only way to undo it is to change each one back by hand or clear the site's localStorage in devtools.

I'd like to add a "Reset all settings" option at the bottom of /settings.

Additional information

• Would you be willing to help implement this feature?

Final checks

• Read the contribution guide.
• Check existing issues.