fix: add Unit.maxHealth() missing from cherry-picked warship decay

The doomsday-clock warship decay commit (#4499) calls ws.maxHealth(),
which was introduced on main by the warship veterancy PR (#4433) — not
present on this branch — breaking the build.

Add maxHealth() to the Unit interface and UnitImpl, returning the
unit-info base (info().maxHealth ?? 1), the same value modifyHealth
already caps against. No veterancy bonus here since this branch lacks
the veterancy system; merging with main will pick up the adjusted
version with only a one-line conflict in the method body.

src/core/game/Game.ts

@@ -479,6 +479,8 @@ export interface Unit {
   transportShipState(): TransportShipState;
   updateTransportShipState(update: Partial<TransportShipState>): void;
   health(): number;
+  /** Max health from unit info (1 for units without health). */
+  maxHealth(): number;
   modifyHealth(delta: number, attacker?: Player): void;
 
   // Troops

src/core/game/UnitImpl.ts

@@ -185,6 +185,9 @@ export class UnitImpl implements Unit {
   health(): number {
     return Number(this._health);
   }
+  maxHealth(): number {
+    return this.info().maxHealth ?? 1;
+  }
   hasHealth(): boolean {
     return this.info().maxHealth !== undefined;
   }