Make feedback form available to anonymous users

**Description:**
The feedback form is currently limited to logged-in users, based on the beta-user assumption. Extend it to support public, logged-out users while keeping basic abuse protection.

**Acceptance criteria:**

- Logged-out users can access and submit the feedback form.
- Anonymous users see an optional email field to provide contact details.
- Logged-in users’ existing identity data is used automatically; they do not need to enter an email.
- Anonymous submissions use a session-based identifier for rate limiting.