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>

api/corpus-summary.php

@@ -0,0 +1,45 @@
+<?php
+declare(strict_types=1);
+/**
+ * GET /api/corpus-summary.php
+ * Returns: { doc_count: int, last_updated: string|null }
+ *
+ * Auth: SSO session only. Ensures dashboard tenant is provisioned.
+ */
+require_once __DIR__ . '/../includes/bootstrap.php';
+
+header('Content-Type: application/json');
+header('Cache-Control: no-store');
+
+if (!dbnToolsIsAuthenticated() || !dbnToolsIsFreeTier()) {
+    http_response_code(401);
+    echo json_encode(['error' => 'auth_required']);
+    exit;
+}
+
+try {
+    $tenant = dbnToolsEnsureDashboardTenant();
+} catch (Throwable $e) {
+    http_response_code(503);
+    echo json_encode(['error' => 'tenant_unavailable']);
+    exit;
+}
+
+try {
+    $db = dbnToolsDb();
+    $stmt = $db->prepare(
+        "SELECT COUNT(*) AS doc_count, MAX(created_at) AS last_updated
+           FROM client_documents
+          WHERE client_id = ? AND status = 'ready'"
+    );
+    $stmt->execute([(int)$tenant['client_id']]);
+    $row = $stmt->fetch(PDO::FETCH_ASSOC);
+
+    echo json_encode([
+        'doc_count'    => (int)($row['doc_count'] ?? 0),
+        'last_updated' => $row['last_updated'] ?? null,
+    ]);
+} catch (Throwable $e) {
+    http_response_code(500);
+    echo json_encode(['error' => 'db_error']);
+}