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Deep-linking to non-root URLs like /settings or /notification-trackers
returned {"detail":"Not Found"} because StaticFiles(html=True) only
serves index.html for directory roots, not for arbitrary SPA routes.
Subclass StaticFiles with an SPA fallback: any 404 on a non-/api path
serves the root index.html, letting the SvelteKit router hydrate the
correct view on the client. Real /api/* 404s still bubble up as JSON
from FastAPI.