Navigation and routing

This skill owns app navigation with go_router. There is exactly ONE GoRouter for the app, defined in lib/routing/, and every screen is reachable by a URL. Navigation is a data structure (routes + a pure redirect), not a pile of imperative Navigator.push calls.

Read the reference for the task at hand:

• references/go-router-config.md — the single router, context.go vs context.push, path params vs state.extra, typed route helpers, errorBuilder/404.
• references/guards-and-redirects.md — pure redirect functions, the Riverpod refreshListenable, auth + onboarding gates, avoiding redirect loops.
• references/shells-and-deep-links.md — StatefulShellRoute.indexedStack for bottom-nav/rail shells, CustomTransitionPage, PopScope, and notification-payload → location mapping.

Run scripts/check_routing.sh before a PR.

Non-negotiable rules