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.

translations/messages+intl-icu.nl.xliff

@@ -369,6 +369,10 @@
         <source>Forgot your password?</source>
         <target>Wachtwoord vergeten?</target>
       </trans-unit>
+      <trans-unit id="3cWWP_q" resname="Fullscreen">
+        <source>Fullscreen</source>
+        <target>Volledig scherm</target>
+      </trans-unit>
       <trans-unit id="MebBrmp" resname="Gray">
         <source>Gray</source>
         <target>Grijs</target>