Adds BattleXpConfig ([ConfigSection("BattleXp")]) with global
XpPerWin/XpPerLoss plus per-mode nullable overrides. Adds BattleXpMode
enum and IViewerRepository.LoadForBattleXpGrantAsync — focused tracked
load with viewer.Classes + Class nav ref included, for the service
about to be introduced.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>