New: record matches with guest players

You can now log a match against someone who doesn't have a TCGCore account yet. On a playgroup's Matches page, any player slot now has a "+ Add new guest..." option — just type their name, and that's it. No account, no invite, no playgroup membership needed. That name is remembered for the playgroup, so the next time you record a match with the same person, you can just pick them from the list instead of typing their name again. This works for regular 1v1 matches and for multiplayer Commander pods alike, so you can mix real members and guests freely in the same match. Once that person actually creates an account and joins the playgroup for real, an owner or admin can link their guest name to their new account. Head to the Participants page, find the new "Guest players" section, and link the name to the right member. Every match recorded under that guest name moves over to their profile from that point on, so their stats, leaderboard position, and match history all catch up at once.