Restore HP/MP to max on level up

[Copilot]

HP and MP should reset to 100% when a player levels up, matching original MapleStory behavior.

• channel/player.go: In levelUp(), set HP/MP to new max values instead of partially healing by the gain amount

// Before: partial heal
newHP := int16(math.Min(float64(newMaxHP), float64(d.hp+hpGain)))
newMP := int16(math.Min(float64(newMaxMP), float64(d.mp+mpGain)))

// After: full restore
d.setHP(newMaxHP)
d.setMP(newMaxMP)