fix(audit): reach system-audit export via target path, not route path

The export dropdown built URLs via lurl('admin/system-audit/export') which
hit a framework quirk: MintyPHP Router::applyRoutes() compares the full
request URI (including ?query) against registered source paths. When
a browser fetched /de/admin/system-audit/export?format=csv the rewrite
to audit/system-audit/export silently missed, the router fell back to
file-based resolution, found no such file under admin/, and redirected
back to the list page — the user saw the grid "reload" and nothing
downloaded.

Only audit is affected because it is the only module where the route
path (admin/…) differs from the page target path (audit/…); helpdesk
and admin/users route paths equal their targets so file-based routing
catches the export URL even when applyRoutes misses.

Fix: use lurl('audit/system-audit/export') in the page config — the
target path — which resolves directly via file-based routing regardless
of query string. This matches the convention the same JS already uses
for `dataUrl: 'audit/system-audit/data'`. Short inline comment records
why.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

modules/audit/pages/audit/system-audit/index(default).phtml

@@ -46,7 +46,11 @@ $pageConfig = [
   'filterSchema' => $clientFilterSchema,
   'filterChipMeta' => $filterChipMeta,
   'gridLang' => $gridLang,
-  'exportUrl' => lurl('admin/system-audit/export'),
+  // Uses the page target path, not the registered route path: the Router's
+  // applyRoutes() rewrite does not strip query strings before matching, so
+  // "admin/system-audit/export?format=csv" would miss the rewrite and 404.
+  // Matches the convention already used for `dataUrl: 'audit/system-audit/data'`.
+  'exportUrl' => lurl('audit/system-audit/export'),
   'labels' => [
     'created' => t('Created'),
     'status' => t('Status'),