Enhancement: Fix post-signup redirect based on user origin (Forum or Aletheia)

### Background Information

## Description

This issue aims to make the sign-up flow more consistent by identifying the user's origin — whether they originated from Aletheia or the Forum.

Regardless of the origin, the registration process will always take place via Aletheia. However, upon completing the sign-up, the user should be redirected back to the platform they originally came from:

- If the user originated from the Forum, they should be redirected to the Forum home.
- If the user originated from Aletheia, they should be redirected to the Aletheia home.

This flowchart represents the user flow originating from the Forum.

This flowchart represents the user journey originating from the Forum.
https://www.figma.com/design/J6OY3ZQILdR7H1qCWyqVw7/Aletheia?node-id=6573-10377&t=xwLvan2NZZlm6oXA-0

### Improvement Goal:

- Provide a smoother user experience by returning users to the appropriate platform after sign-up.
- Maintain context awareness during cross-platform authentication flows.
- Avoid user confusion caused by being redirected to an unrelated platform.

### Steps to Reproduce

1. Access the Forum.
2. Click on "Login", then select "Sign Up".
3. Complete the registration process on the Aletheia sign-up page.
4. After signing up, observe that the user is redirected to the Aletheia home page, instead of being redirected back to the Forum, which was the original entry point.

### How

## Acceptance Criteria:

- The system captures the origin of the user before redirecting to the Aletheia sign-up.
- A `redirect_to` parameter or equivalent mechanism is implemented to store the origin.
- After completing the sign-up, the user is redirected to the correct platform based on origin:
[ ] Forum → Forum home
[ ] Aletheia → Aletheia home
- The behavior is validated for both login and sign-up flows.
- The implementation handles edge cases where the origin is missing or invalid.

### Expected Behavior

After completing the sign-up process:

- If the user originated from the Forum, they should be redirected to the Forum home page.
- If the user originated from Aletheia, they should be redirected to the Aletheia home page.

The system should preserve the origin context throughout the sign-up flow to ensure a coherent navigation experience.