Smoother user data sync and hydration across sessions

User data syncing got a thorough rework so the experience across multiple tabs, devices and reloads feels coherent:

• Cross-tab sync. Signing in or updating your profile in one tab now propagates to other open tabs without a refresh.
• First-paint hydration. The flicker between "guest" and "authenticated" states on first paint is gone — the app now decides upfront and stays consistent.
• Session restoration. Restoring a session from a cookie no longer momentarily lands you on a logged-out view.

None of these were dramatic individually, but they were the kind of micro-jank that adds up to making the app feel slightly off. Things should now feel solid.