reports page: spec-driven generate panel, list, and sliceable viewer

Add a /reports page built on react-openapi's runtime configs:
- inline generate panel (GroupSpec enum, prefetched payee picker,
  strict DD-MM-YYYY range, 400 surfacing)
- metadata report list with view/regenerate/delete
- viewer that slices the cached per-entity payload via
  metadata.group_options, aggregates metrics + txns, pivots
  period x payee, and renders month accordions
- route + nav entry; no hand-rolled schema duplication

src/Header.tsx

@@ -34,6 +34,7 @@ interface HeaderProps {
 const NAV_LINKS = [
   { label: "Home", path: "/" },
   { label: "Expenses", path: "/expenses" },
+  { label: "Reports", path: "/reports" },
   { label: "Fetch Requests", path: "/fetch-requests" },
 ];