feat(auth): Google OAuth, sealed session and default-deny gating (T9)

Gate the app behind Google sign-in restricted to verified @theodo.com
identities, and record the real reporter on filed defects (F9).

- nuxt-auth-utils for sealed cookie sessions + the Google OAuth handler,
  mounted at /auth/google/callback to match the registered redirect URI.
- isAllowedGoogleUser re-derives the domain from Google's verified email;
  the spoofable `hd` claim is deliberately ignored (DESIGN T9).
- Default-deny: server middleware 401s unauthenticated /api calls (health
  and the session endpoint excepted); a global route middleware redirects
  unauthenticated page navigations to /login.
- getReporter() now reads the session email instead of the dev stub.
- Env contract moves to nuxt-auth-utils names (NUXT_SESSION_PASSWORD,
  NUXT_OAUTH_GOOGLE_*); .env.example, compose and README updated.

Unit-tested: domain check (incl. hd-spoof + look-alike) and public-path
matching. Live OAuth round-trip pending manual verification.

app/layouts/default.vue

@@ -1,6 +1,12 @@
 <script setup lang="ts">
 // Single domain (ADR 0004): report at `/`, view all defects at `/defects`,
 // with a persistent header to move between them.
+const { loggedIn, user, clear } = useUserSession()
+
+async function signOut() {
+  await clear()
+  await navigateTo('/login')
+}
 </script>
 
 <template>
@@ -25,6 +31,12 @@
             </NuxtLink>
           </li>
         </ul>
+        <div v-if="loggedIn" class="flex items-center gap-3 pl-2">
+          <span class="hidden text-sm opacity-70 sm:inline">{{ user?.email }}</span>
+          <button type="button" class="btn btn-ghost btn-sm" @click="signOut">
+            Sign out
+          </button>
+        </div>
       </nav>
     </header>