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>

api/guest-session.php

@@ -0,0 +1,11 @@
+<?php
+declare(strict_types=1);
+require_once __DIR__ . '/../includes/bootstrap.php';
+dbnToolsStartSession();
+$_SESSION['dbn_tools_guest_ok'] = true;
+$raw    = trim((string)($_GET['return'] ?? '/dashboard.php'));
+$return = (str_starts_with($raw, '/') && !str_starts_with($raw, '//') && !preg_match('/[\r\n]/', $raw))
+    ? $raw
+    : '/dashboard.php';
+header('Location: ' . $return);
+exit;