feat(settings): toggle visibility (public/private)

[Rend1027]

Allow users to choose whether their listening activity (geolocation + track) is publicly shown on:

• The map
• The dashboard
• Trending analytics
  Users who toggle visibility off will still use the app, but won’t appear in any public views.

Front-End

• Create toggle input in settings page
• Connect to backend using PATCH request
• Show a success message or update UI

Back-End

• Add is_public to User model (default: true)
• Create a PATCH route to update visibility
• Update session queries to filter users where is_public = true

🔒 Respecting user visibility is essential for trust

🎯 We should test toggling OFF and verify the user disappears from the dashboard/map

🧪 Use mock users to test both public and private cases

⏳ Later, we can prompt users with a modal post-login to set this preference