Multi-site fleet page — clean up deprecated /sites and /settings/sites routes

[flesher]

Follow-up to #368 (Fleet page scaffold).

Context

The 2026-06-02 redesign collapses /sites (operational overview) and /settings/sites (config surface) into /fleet/sites + /sites/:id. The old routes still exist for direct-URL access during the transition but the sidenav entries are gone.

Scope

Once #373 (/sites/:id enrichment) is feature-complete and operators have moved over:

• Convert /sites and /settings/sites to permanent redirects to /fleet/sites.
• Delete SitesPage.tsx, SettingsSitesPage.tsx, and SitesAllTable.tsx (replaced by SitesListTable).
• Delete ManageSiteModal and its tests (functionality moved to SiteDetailsModal + /sites/:id).
• Drop importSitesPage + importSettingsSitesPage from routePrefetch.ts.
• Update plan doc references.

Notes

• This is a cleanup pass — defer until Multi-site fleet page — /sites/:id detail page enrichment (metrics row, Details table, buildings grid) #373 ships and traffic has actually moved. Don't break dogfood links prematurely.
• See "TDD reconciliation" in the plan doc for the salvage-vs-abandon analysis of pre-redesign TDDs.