refactor: replace useSyncExternalStore with useState + useEffect for route transitions

[uhyo]

Summary

• Replaces useSyncExternalStore with useState + useEffect for subscribing to location changes in the <Router> component
• This change enables future support for route transitions (e.g., startTransition), since useSyncExternalStore forces synchronous re-renders that cannot be wrapped in transitions
• Handles SSR by checking typeof window for initial state, and syncs with the client snapshot on mount to cover SSR-to-hydration mismatches

Test plan

• Run pnpm test:run to verify all existing tests pass
• Run pnpm typecheck to verify type correctness
• Test the example app (pnpm --filter funstack-router-example dev) and verify navigation still works correctly

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