feature: Add experimental feature opt-ins to profile settings

[joshsadam]

What does this PR do and why?

This branch adds a Profile > Experimental Features page so users can opt in and out of enabled experimental features themselves.

It includes:

• profile routing, controller actions, views, and Turbo Stream updates for feature toggles;
• a Stimulus toggle flow that keeps keyboard focus on the updated control after each change;
• config-driven, localized feature names and descriptions;
• service-level enforcement for restricted-feature allowlists and stricter opt-in validation;
• persistence of user opt-ins in ApplicationSetting via user_opt_in_features;
• expanded coverage across controller, service, model, and system tests.

How to set using the Rails Console:

app = ApplicationSetting.current || ApplicationSetting.create_from_defaults
patch = {
  "data_grid_samples_table" => {
    "name" => { "en" => "New EN Name" }
  }
}
merged = (app.user_opt_in_features || {}).deep_merge(patch)
app.update!(user_opt_in_features: merged)
app.reload.user_opt_in_features

OR a new feature flag:

bin/rails c
app_setting = ApplicationSetting.current || ApplicationSetting.create_from_defaults

default_opt_in_features = {
  "client_linelist_exports_v1" => {
    "allowlist" => ["user1@email.com"],
    "name" => {
      "en" => "Client Side Linelist Export",
      "fr" => "Tableau de données des échantillons"
    },
    "description" => {
      "en" => "Enable the new client side data export.",
      "fr" => "Activer la nouvelle grille de données pour le tableau d'échantillons."
    }
  }
}

app_setting.update!(
  user_opt_in_features: default_opt_in_features.merge(app_setting.user_opt_in_features || {})
)
app.reload.user_opt_in_features

Screenshots or screen recordings

If there are experimental features to toggle

If there are not experimental features to toggle

How to set up and validate locally

1. Run bin/setup --reset to refresh DB + seeds.
2. Start the app.
3. Sign in as user1@email.com and open Profile > Experimental Features.
4. Confirm each visible feature shows the expected localized name/description.
5. Toggle a feature on and off, and confirm row-level Turbo updates (no full page reload).
6. Confirm keyboard focus returns to the updated toggle after each change.
7. Verify allowlisted users can opt in to restricted features and non-allowlisted users cannot.
8. Run:
   PARALLEL_WORKERS=4 bundle exec rails test test/controllers/profiles/experimental_features_controller_test.rb
9. Run:
   PARALLEL_WORKERS=4 bundle exec rails test test/services/profiles/experimental_features/opt_in_service_test.rb
10. Run:
    PARALLEL_WORKERS=4 bundle exec rails test test/models/application_setting_test.rb
11. Run:
    PARALLEL_WORKERS=4 bundle exec rails test test/system/profile_test.rb -n "/experimental features with keyboard and keep focus/"

PR acceptance checklist

• I have evaluated the PR acceptance checklist for this PR.

[deepsidhu85]

Just a quick thing after a quick glance below. Also I think we should split this PR up into smaller pieces

[deepsidhu85]

I think we should refactor most of the code in the controller into using a service such Profiles::ExperimentalFeatures::UpdateService. Controller should just be making a call to the service and returning a result, just to keep the controller light

[joshsadam]

Updated in 0fb966b.