State persistence

The `P2PHost` currently uses [an in-memory storage implementation](https://github.com/boardgameio/p2p/blob/f84a73aad94e19a564de0e3a5f0d98dd0ad2b498/src/db.ts) to hold match state. This means navigating, closing a tab, or just stopping/hiding the client will usually result in that state being lost.

We should use a persistent storage layer (e.g. using `localStorage` or `indexedDB`) and offer some options for how long matches should persist for as well as some way to delete matches.

Start with the linked src/db.ts implementation to understand how P2PHost currently holds match state in memory. Define the persistence behavior, match-retention options, and deletion mechanism, then verify that state survives navigation, closing, stopping, or hiding the client and that matches can be removed.

Written by the indexing model from the issue text.