Prevent Flag Defaults: UI enforcement lost in create feature modal refactor

[Holmus]

Description

The "Prevent Flag Defaults" project setting no longer hides the value and enabled fields when creating a feature flag. Users can set a value and enabled state during creation, even though the setting description says it "forces the user to create a feature before setting its values per environment".

The backend still works correctly (other environments get enabled=false and no value), but the frontend no longer enforces the intended UX of hiding those fields during creation.

Regression

This was introduced in #6356 (commit 00085cd, 20 Jan 2026). The old CreateFlag.js component checked project.prevent_flag_defaults to:

1. Hide the value/enabled fields during feature creation
2. Show an info message: "This will create the feature for all environments, you can edit this feature per environment once the feature is created."

During the refactor to the new create-feature/tabs/CreateFeature.tsx, the hideValue prop was hardcoded to false (line 140) and the prevent_flag_defaults check was dropped.

Expected behaviour

When "Prevent Flag Defaults" is enabled on a project, the create feature form should not show value or enabled state fields - matching the previous behaviour and the setting description.

Steps to reproduce

1. Enable "Prevent Flag Defaults" in Project Settings > Additional Settings
2. Create a new feature flag
3. Observe that value and enabled fields are still visible and editable