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>

includes/layout_footer.php

@@ -47,5 +47,60 @@
     </menu>
   </form>
 </dialog>
+
+<?php if (!empty($layoutIsGuest)): ?>
+<!-- Auth gate modal — shown to unauthenticated visitors unless they dismissed it -->
+<div id="authGate" class="auth-gate-backdrop" hidden aria-modal="true" role="dialog" aria-labelledby="authGateTitle">
+    <div class="auth-gate-card">
+        <p class="auth-gate-eyebrow">Do Better Norge</p>
+        <h2 id="authGateTitle" class="auth-gate-title">Logg inn for å bruke verktøyene</h2>
+        <p class="auth-gate-body">Våre juridiske AI-verktøy krever en gratis konto. Registrer deg på sekunder med Google.</p>
+        <div class="auth-gate-actions">
+            <a id="authGateLogin" href="/?return=<?= htmlspecialchars($layoutReturnUrl ?? '') ?>" class="auth-gate-btn auth-gate-btn--primary">Logg inn / Registrer deg</a>
+            <button id="authGateDismiss" class="auth-gate-btn auth-gate-btn--ghost">Fortsett uten konto</button>
+        </div>
+        <p class="auth-gate-note">Gratis tilgang · Ingen kredittkort</p>
+    </div>
+</div>
+<script>
+(function () {
+    'use strict';
+    var GATE_KEY   = 'dbn-auth-gate-v1';
+    var BANNER_KEY = 'dbn-guest-banner-v1';
+    var gate       = document.getElementById('authGate');
+    var banner     = document.getElementById('guestBanner');
+    var bannerClose = document.getElementById('guestBannerClose');
+
+    // Show or hide the guest banner
+    if (banner) {
+        if (localStorage.getItem(BANNER_KEY) === 'closed') {
+            banner.hidden = true;
+        }
+        if (bannerClose) {
+            bannerClose.addEventListener('click', function () {
+                localStorage.setItem(BANNER_KEY, 'closed');
+                banner.hidden = true;
+            });
+        }
+    }
+
+    // Show auth gate modal unless already dismissed
+    if (gate && localStorage.getItem(GATE_KEY) !== 'dismissed') {
+        gate.hidden = false;
+        document.getElementById('authGateDismiss').addEventListener('click', function () {
+            localStorage.setItem(GATE_KEY, 'dismissed');
+            gate.hidden = true;
+        });
+        // Close on backdrop click
+        gate.addEventListener('click', function (e) {
+            if (e.target === gate) {
+                localStorage.setItem(GATE_KEY, 'dismissed');
+                gate.hidden = true;
+            }
+        });
+    }
+}());
+</script>
+<?php endif; ?>
 </body>
 </html>