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/client/graphics/layers/NameLayer.ts

@@ -461,7 +461,7 @@ export class NameLayer implements Layer {
       );
     });
     const isMyPlayerTarget = nukesSentByOtherPlayer.find((unit) => {
-      const detonationDst = unit.detonationDst();
+      const detonationDst = unit.targetTile();
       if (detonationDst === undefined) return false;
       const targetId = this.game.owner(detonationDst).id();
       return myPlayer && targetId === myPlayer.id();

src/client/graphics/layers/UnitLayer.ts

@@ -291,7 +291,7 @@ export class UnitLayer implements Layer {
   }
 
   private handleWarShipEvent(unit: UnitView) {
-    if (unit.warshipTargetId()) {
+    if (unit.targetUnitId()) {
       this.drawSprite(unit, colord({ r: 200, b: 0, g: 0 }));
     } else {
       this.drawSprite(unit);
@@ -502,7 +502,7 @@ export class UnitLayer implements Layer {
 
     if (this.alternateView) {
       let rel = this.relationship(unit);
-      const dstPortId = unit.dstPortId();
+      const dstPortId = unit.targetUnitId();
       if (unit.type() === UnitType.TradeShip && dstPortId !== undefined) {
         const target = this.game.unit(dstPortId)?.owner();
         const myPlayer = this.game.myPlayer();