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_dashboard.php

@@ -37,6 +37,13 @@ $dashboardTitle  = $dashboardTitle  ?? 'Dashboard';
 $dashboardLead   = $dashboardLead   ?? '';
 $langPath        = strtok((string)($_SERVER['REQUEST_URI'] ?? '/dashboard/'), '?') ?: '/dashboard/';
 
+$dashAuthUser    = dbnToolsAuthenticatedUser();
+$dashUserDisplay = '';
+if ($dashAuthUser !== null) {
+    $email = (string)($dashAuthUser['email'] ?? '');
+    $dashUserDisplay = strstr($email, '@', true) ?: $email;
+}
+
 $dashboardNav = [
     'index'     => ['url' => '/dashboard/',           'label' => 'Oversikt',   'sub' => 'Overview'],
     'documents' => ['url' => '/dashboard/documents.php', 'label' => 'Dokumenter', 'sub' => 'Documents'],
@@ -78,6 +85,12 @@ window.DBN_DASHBOARD = {
         <nav class="dash-topbar__tools" aria-label="Tools">
             <a href="/dashboard.php" class="dash-topbar__link">← Tilbake til verktøy</a>
         </nav>
+        <div class="dash-topbar__user">
+            <?php if ($dashUserDisplay !== ''): ?>
+            <span class="dash-topbar__username" title="<?= htmlspecialchars($dashAuthUser['email'] ?? '') ?>"><?= htmlspecialchars($dashUserDisplay) ?></span>
+            <?php endif; ?>
+            <a href="/api/logout.php" class="dash-topbar__logout">Logg ut</a>
+        </div>
     </header>
 
     <div class="dash-layout">