Add fullscreen toggle and return-to-page logout redirect (#200)

* feat: add fullscreen toggle and return-to-page logout redirect

Add a Stimulus-driven fullscreen button/keypress to the quiz and
elimination screens, and redirect admins back to the page they were on
after logging out (unless it was a backoffice or elimination page).

* docs: expand CLAUDE.md domain context and testing conventions

Document the WIDM quiz/elimination domain model in more depth, and add
guidance to prefer plain TestCase over WebTestCase/KernelTestCase and to
apply the Boy Scout Rule for small nearby cleanups.

* fix: drop f-keypress fullscreen shortcut

Candidate names can start with "f" while typing, so the keypress
conflicted with the enter-name form. Button-only toggle remains.

templates/quiz/nav.html.twig

@@ -4,7 +4,7 @@
             <a href="{{ path('tvdt_backoffice_index') }}" class="btn btn-outline-secondary btn-sm">
                 {{ 'Backoffice'|trans }}
             </a>
-            <a href="{{ path('tvdt_login_logout') }}" class="btn btn-outline-secondary btn-sm">
+            <a href="{{ path('tvdt_login_logout', {target: app.request.pathInfo}) }}" class="btn btn-outline-secondary btn-sm">
                 {{ 'Logout'|trans }}
             </a>
         {% else %}