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/controllers/fullscreen_controller.js

@@ -0,0 +1,40 @@
+import {Controller} from '@hotwired/stimulus';
+
+const STORAGE_KEY = 'tvdt-fullscreen';
+
+export default class extends Controller {
+  connect() {
+    this.onFullscreenChange = this.onFullscreenChange.bind(this);
+    document.addEventListener('fullscreenchange', this.onFullscreenChange);
+    this.syncState();
+
+    if (sessionStorage.getItem(STORAGE_KEY) === '1' && !document.fullscreenElement) {
+      this.request();
+    }
+  }
+
+  disconnect() {
+    document.removeEventListener('fullscreenchange', this.onFullscreenChange);
+  }
+
+  toggle() {
+    if (document.fullscreenElement) {
+      document.exitFullscreen();
+    } else {
+      this.request();
+    }
+  }
+
+  request() {
+    document.documentElement.requestFullscreen().catch(() => {});
+  }
+
+  onFullscreenChange() {
+    sessionStorage.setItem(STORAGE_KEY, document.fullscreenElement ? '1' : '0');
+    this.syncState();
+  }
+
+  syncState() {
+    document.documentElement.classList.toggle('is-fullscreen', Boolean(document.fullscreenElement));
+  }
+}