feat: add MaxManaBonusRule and initiative mana recovery

[trevlar]

Summary

• Add MaxManaBonusRule - a new rule type that adds to max mana (follows MaxHpBonusRule pattern)
• Implement Initiative Mana Recovery - grants mana when rolling initiative for classes with recovery: 'initiative', cleared when combat ends
• Update rest logic to exclude initiative-only recovery from rest restoration

Test plan

• Create an item with a maxManaBonus rule, assign to character, and verify max mana increases
• Remove item and verify max mana decreases
• Create a class with mana.recovery: 'initiative' and a mana formula
• Add character with this class to combat, roll initiative, and verify mana is granted
• End combat and verify combat-granted mana is cleared
• With initiative-only class, take a rest and verify mana is NOT restored