To avoid unintended resource transports to other player, i think about a check on fleet page III

Own coordinates:
- Always possible, no hint

Own alliance:
- Always possible, showing hint

Other active player:
- Possible after confirmation, showing hint
-- The Send button grey/disabled, can be enabled with a click

Inactive player:
- Possible after confirmation, showing hint
-- The Send button grey/disabled, can NOT be enabled with a click
-- The hint includes a checkbox to enable the send button