Gate JoinPlanetAttack on having >=1 charge (when PwAttackChargesMax > 0
in DynamicConfig). Attackers spend 1 charge when their squad actually
launches (LaunchAllBattles), so merely clicking without forming a squad
does not cost anything. Defenders gain 1 charge at battle end (via
ProcessBattleResult). Passive recharge: a player who has sat below max
for PwAttackChargesRechargeMinutes gains 1 charge; fresh/zero-initialised
accounts get their recharge timer set lazily on the first passive pass.

PwCharges protocol message (Current, Max, SecondsUntilNextCharge) is
pushed to the player on login and after every change. Max=0 disables
the system. StartGalaxy seeds every account to the configured maximum.