feat: replace all hardcoded constants with config values

Replace hardcoded game constants with values from the config system:
- GameSession now receives *config.Config from Lobby
- TurnTimeout, MaxFloors, SkillUses, InventoryLimit use config values
- combat.AttemptFlee accepts fleeChance param
- combat.ResolveAttacks accepts coopBonus param
- entity.NewMonster accepts scaling param
- Solo HP/DEF reduction uses config SoloHPReduction
- Lobby JoinRoom uses config MaxPlayers

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/model.go

@@ -405,7 +405,7 @@ func (m Model) updateClassSelect(msg tea.Msg) (tea.Model, tea.Cmd) {
 				room := m.lobby.GetRoom(m.roomCode)
 				if room != nil {
 					if room.Session == nil {
-						room.Session = game.NewGameSession()
+						room.Session = game.NewGameSession(m.lobby.Cfg())
 					}
 					m.session = room.Session
 					player := entity.NewPlayer(m.playerName, selectedClass)