test: Add Vitest structural conformance tests for aligned pages (#1626)

Verifies that AccountsPage, PartiesPage, and PaymentsPage each render
a PageShell wrapper (div.space-y-6) and a PageHeader h1 with canonical
styling (text-3xl font-bold tracking-tight).

Verifies that AccountDetailPage, PartyDetailPage, and PaymentDetailPage
each render Breadcrumbs, show a loading skeleton, and show an error
state on fetch failure.

Co-authored-by: Ben Coombs <bjcoombs@users.noreply.github.com>

Author: bjcoombs