Clean up and refactor the Unit class (#769)

## Description:

* Merged similar fields so they can be reused (eg warshipTarget =>
targetUnit)
* simplified isCooldown api
* added "touch" method to send update to UI layer
* standardized on "undefined"


## Please complete the following:

- [ ] I have added screenshots for all UI updates
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [x] I understand that submitting code with bugs that could have been
caught through manual testing blocks releases and new features for all
contributors

## Please put your Discord username so you can be contacted if a bug or
regression is found:

<DISCORD USERNAME>
evan

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **Refactor**
- Unified and simplified how unit targets and cooldowns are managed
across all unit types, resulting in more consistent in-game behavior for
nukes, warships, trade ships, and SAM launchers.
- Updated naming and logic for unit targeting and cooldowns, improving
clarity in status displays and interactions.
- Reorganized unit interface and streamlined cooldown handling for
smoother gameplay experience.
- **Bug Fixes**
- Corrected visual indicators for nukes and warships to accurately
reflect their targets.
- **Tests**
- Updated automated tests to align with the new cooldown and targeting
logic.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

src/core/execution/TradeShipExecution.ts

@@ -94,7 +94,7 @@ export class TradeShipExecution implements Execution {
         return;
       } else {
         this._dstPort = ports[0];
-        this.tradeShip.setDstPort(this._dstPort);
+        this.tradeShip.setTargetUnit(this._dstPort);
       }
     }
 
@@ -122,7 +122,7 @@ export class TradeShipExecution implements Execution {
         break;
       case PathFindResultType.Pending:
         // Fire unit event to rerender.
-        this.tradeShip.move(this.tradeShip.tile());
+        this.tradeShip.touch();
         break;
       case PathFindResultType.NextTile:
         this._dstPort.cachePut(this.tradeShip.tile(), result.tile);