Add hideContentOnSubmit option to SubmitButton component

[Copilot]

This PR adds a configurable hideContentOnSubmit option to the SubmitButton component that allows developers to control whether page content is hidden after submission.

Problem

Previously, when participants clicked a submit button, the page content was always hidden and replaced with a "Please wait for other participant(s)." message. However, sometimes we want participants to be able to indicate that they are ready to proceed without hiding the page content, allowing them to continue viewing and interacting with the material.

Solution

Added a new boolean prop hideContentOnSubmit to the SubmitButton component with the following behavior:

• Default (backwards compatible): When hideContentOnSubmit is not specified or set to true, content is hidden after submission (preserves original behavior)
• New functionality: When hideContentOnSubmit: false, page content remains visible after submission

Implementation Details

Server-side Schema Validation

• Updated validateTreatmentFile.ts to accept the new hideContentOnSubmit field as an optional boolean with default value true

Client-side Changes

• SubmitButton component: Added hideContentOnSubmit prop and stores the preference on the player object when clicked
• Element component: Passes the hideContentOnSubmit value from element configuration to SubmitButton
• SubmissionConditionalRender: Modified to check the stored flag and only hide content when hideContentOnSubmit !== false

Usage Examples

# Default behavior (content hidden after submit)
- type: submitButton
  buttonText: Continue

# Explicit hide content
- type: submitButton
  buttonText: Continue  
  hideContentOnSubmit: true

# Keep content visible (NEW)
- type: submitButton
  buttonText: I'm Ready
  hideContentOnSubmit: false

Testing

• Added comprehensive test treatment with three scenarios covering all use cases
• Created dedicated Cypress test file to validate functionality
• Verified backwards compatibility and proper default handling

Fixes #675.

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