feat(nav): unified navbar, account page, corpus summary widget, and i18n fixes

- New includes/nav.php: sticky site-wide nav with Tools dropdown, Dashboard
  link, compact language switcher, user identity → /account.php, Log out
- New account.php: credits & plan, profile, team, usage sections
- New api/corpus-summary.php: JSON endpoint for corpus doc count + last updated
- Replaces topbar in layout.php, layout_dashboard.php, and dashboard.php
- Fixes hardcoded Norwegian strings in dashboard.php credit cards via dbnToolsT()
- Adds 35 new i18n keys across all 4 languages (en/no/uk/pl) in i18n.php
- CSS: .dbn-nav navbar + .account-* account page styles in tools.css

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

includes/layout_dashboard.php

@@ -73,25 +73,8 @@ window.DBN_DASHBOARD = {
 };
 </script>
 
+<?php include __DIR__ . '/nav.php'; ?>
 <div class="dash-shell">
-    <header class="dash-topbar" role="banner">
-        <a class="dash-brand" href="/dashboard/">
-            <span class="dash-brand__mark">⚖</span>
-            <span class="dash-brand__text">
-                <strong>Min korpus</strong>
-                <small>Do Better Norge</small>
-            </span>
-        </a>
-        <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">
         <nav class="dash-sidebar" aria-label="Dashboard sections">