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.

assets/styles/quiz.scss

@@ -94,6 +94,32 @@ input.btn-check:checked + label.answer-btn {
     gap: 0.5rem;
 }
 
+.fullscreen-btn {
+    position: fixed;
+    bottom: 0.75rem;
+    left: 0.75rem;
+    z-index: 1040;
+    width: 2.25rem;
+    height: 2.25rem;
+    padding: 0;
+    background: transparent;
+    border: none;
+    color: rgba(255, 255, 255, 0.35);
+    font-size: 1.25rem;
+    line-height: 1;
+    cursor: pointer;
+
+    &:hover,
+    &:focus {
+        color: rgba(255, 255, 255, 0.8);
+        outline: none;
+    }
+}
+
+html.is-fullscreen .fullscreen-btn {
+    display: none;
+}
+
 .elimination-screen {
     position: fixed;
     top: 0;