Newsletter Form: No Submit Behavior

[kOaDT]

Objective

Add a client-side submit handler so that submitting the newsletter form gives immediate user feedback (e.g. thank-you message or toast) without a backend. Form submission must not trigger a full-page navigation.
The idea is just to improve UX perception, but the form submission must not trigger anything. (Except if you want to implement a new vulnerability?)

Scope

• Component: app/components/Newsletter.tsx
• Backend: None. No new API route or persistence.
• Dependencies: Prefer existing patterns; add a toast library only if already used elsewhere.

Acceptance Criteria

1. Submitting the form does not cause a full page reload (default submit prevented).
2. User sees a success message (inline text or toast) after submit.
3. Implementation remains minimal and does not introduce unnecessary dependencies unless consistent with the project.