feat(auth): add login/logout, user identity, and soft auth gate

- api/logout.php: destroys session + clears cookie, redirects to /
- api/guest-session.php: sets guest flag, lets users explore without account
- layout.php: removes hard PHP redirect; authenticated users see email +
  "Logg ut" in topbar; guests see guest banner (sticky, dismissible) and
  auth gate modal (dismissible via localStorage) instead of redirect
- layout_footer.php: injects auth gate modal + JS for banner/modal dismiss
- layout_dashboard.php: adds username + "Logg ut" to dash-topbar
- index.php: adds "Utforsk uten konto" link under primary login CTA
- tools.css: .guest-banner, .auth-gate-*, .topbar-user, .dash-topbar__user

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

index.php

@@ -144,6 +144,7 @@ window.DBN_TOOLS_LANG = <?= json_encode($uiLang, JSON_UNESCAPED_UNICODE) ?>;
                 <h2 class="lt-gate__card-title lt-gate__card-title--large" id="ltGateTitle"><?= htmlspecialchars(dbnToolsT('member_card_title', $uiLang)) ?></h2>
                 <p class="lt-gate__card-note"><?= htmlspecialchars(dbnToolsT('member_card_note', $uiLang)) ?></p>
                 <a class="lt-access-btn" href="<?= htmlspecialchars($toolsLogin) ?>"><?= htmlspecialchars(dbnToolsT('primary_access', $uiLang)) ?></a>
+                <p class="lt-gate__explore-link"><a href="/api/guest-session.php?return=/dashboard.php">Utforsk uten konto &rarr;</a></p>
                 <details class="fallback-login">
                     <summary><?= htmlspecialchars(dbnToolsT('secondary_access', $uiLang)) ?></summary>
                     <form id="passcodeForm" class="passcode-form">