CVE-2026-45161: wger: trainer_login accepts GET - CSRF bypass enables forced session rebinding

The trainer_login view in wger accepts GET requests and executes django_login() without any CSRF protection, because Django’s CsrfViewMiddleware only enforces tokens on unsafe methods (POST/PUT/PATCH/DELETE). An attacker can embed a single <img> tag on a malicious page; when an authenticated trainer loads that page, their browser auto-issues the GET with the session cookie, forcibly rebinding the trainer’s session to an arbitrary user account.