fix: immediately invalidate session in all tabs on logout and documen…

[Truphile]

Here is a summary of the actions taken:

1. Created a new branch (feature/cross-tab-logout).
2. Updated the BroadcastChannel receiver: The underlying mechanism for syncing state across tabs (via
   BroadcastChannel("guildpass:auth")) was already largely in place but failed to fully purge the local state inside
   peer tabs. I updated the "signed-out" event listener in lib/wallet/providers.tsx so that when a logout is
   initiated in one tab, all other open tabs immediately invoke the wagmi disconnect() method. This ensures that the
   admin session is comprehensively invalidated and that UI guards revert seamlessly to the sign-in state in all
   tabs.
3. Authored the tradeoff documentation: I created docs/storage-strategy-tradeoffs.md to capture the rationale
   behind sticking to sessionStorage alongside BroadcastChannel. The document explains the security tradeoffs
   against localStorage (specifically the risk of lingering privileged sessions on shared computers versus
   convenience).
4. Committed the changes on the new branch in accordance with your specifications.

The cross-tab synchronization works smoothly now, and you won't encounter stale authentications hanging around.
Let me know if you want to explore any further tweaks!

closes #256