Consolidate the three hand-synced copies of the migrated-pages map
(LEGACY_REDIRECTS in app/page.tsx, MIGRATED_PAGES in the dashboard layout,
and MIGRATED_PAGES in leftnav) into one shared module,
src/utils/migratedPages.ts, which also owns the migratedHref helper. Delete
the unused, incomplete Sidebar2 prototype.
No runtime behavior change: the map is still empty and Sidebar2 had no
importers, so this is pure deduplication ahead of the per-page App Router
migration. Follow-up work will unify the remaining base-URL builders
(layout's withBase and page.tsx's redirect) onto migratedHref.
ff6cea4833