Arena fights reworked and notification system

The arena fight screen has been reworked to use the new spell system that replaced the old one earlier this week. Spell selection, targeting, and resolution all go through the updated pipeline now — the arena was the last place still on the old path.

A notification system is in place. Instead of blocking popups for every event, the game can now queue lightweight notices that appear and dismiss without pulling you out of what you're doing. Still tuning when to use a notification versus a full popup, but the infrastructure is there.

Tests and coverage were brought back up to the 75% threshold after the arena changes. A few API client tests were added to cover edges that the spell rework exposed.