Frontend — Unsubscribe page

[cka-y]

Description:
Create a new frontend route at /notifications/unsubscribe?id=<UUID>. When a user clicks the unsubscribe link in an email, this page calls the API to deactivate the subscription and shows a confirmation message.

Acceptance Criteria:

• Route /notifications/unsubscribe exists and reads id from query params
• Page calls DELETE /v1/subscriptions/{id} on load or user confirmation
• Shows success confirmation after unsubscription
• Handles error states (invalid UUID, already unsubscribed, network error)