feat: multi-warship selection with Shift+drag box (#3677)

Resolves #3666

## Description:

Adds RTS-style box selection for warships. Hold Shift and drag (desktop)
or long-press and drag (touch/mobile) to draw a selection rectangle —
all player-owned warships inside get selected at once. A subsequent
click/tap on water sends them all to that location.

- `SelectionBoxLayer` — pixel-dashed rectangle in world-space, player
territory color; shared between desktop and touch
- `UILayer` — same pulsing selection outline on each box-selected
warship; clears correctly when switching between single/multi selection
- `UnitLayer` — finds warships in screen rect, filters inactive ships
before sending; touch support included
- `InputHandler` — Shift+drag and touch long-press+drag both emit
selection box events; cursor becomes crosshair on Shift; discards active
ghost structure on Shift press; configurable via `shiftKey` keybind
- `Transport` — single atomic `move_multiple_warships` intent (no split
on socket drop)
- `Schemas` + `ExecutionManager` + `MoveMultipleWarshipsExecution` —
server fans out atomic intent into individual `MoveWarshipExecution` per
ship
- `DynamicUILayer` — `MoveIndicatorUI` chevron animation on target tile
for both single and multi move
- `UnitDisplay` — warship tooltip Shift hint via `translateText`
- `HelpModal` — new hotkey row: Shift + drag → select multiple warships

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

## UI update
### Mouse + Keyboard


https://github.com/user-attachments/assets/3f35ab5e-1f3c-4c5d-bc4f-aabccf64dc60

### Touch


https://github.com/user-attachments/assets/0d6aec3f-44fa-4fee-b5c6-b267b9b14d79

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

fghjk_60845

src/core/execution/ExecutionManager.ts

@@ -68,7 +68,7 @@ export class Executor {
       case "cancel_boat":
         return new BoatRetreatExecution(player, intent.unitID);
       case "move_warship":
-        return new MoveWarshipExecution(player, intent.unitId, intent.tile);
+        return new MoveWarshipExecution(player, intent.unitIds, intent.tile);
       case "spawn":
         return new SpawnExecution(this.gameID, player.info(), intent.tile);
       case "boat":

src/core/execution/MoveWarshipExecution.ts

@@ -4,31 +4,36 @@ import { TileRef } from "../game/GameMap";
 export class MoveWarshipExecution implements Execution {
   constructor(
     private readonly owner: Player,
-    private readonly unitId: number,
+    private readonly unitIds: number[],
     private readonly position: TileRef,
   ) {}
 
-  init(mg: Game, ticks: number): void {
+  init(mg: Game, _ticks: number): void {
     if (!mg.isValidRef(this.position)) {
       console.warn(`MoveWarshipExecution: position ${this.position} not valid`);
       return;
     }
-    const warship = this.owner
-      .units(UnitType.Warship)
-      .find((u) => u.id() === this.unitId);
-    if (!warship) {
-      console.warn("MoveWarshipExecution: warship not found");
-      return;
+    // Cache warship list and build a lookup map — avoids repeated iteration
+    const warshipMap = new Map(
+      this.owner.units(UnitType.Warship).map((u) => [u.id(), u]),
+    );
+    // Deduplicate ids so each warship is only moved once
+    for (const unitId of new Set(this.unitIds)) {
+      const warship = warshipMap.get(unitId);
+      if (!warship) {
+        console.warn(`MoveWarshipExecution: warship ${unitId} not found`);
+        continue;
+      }
+      if (!warship.isActive()) {
+        console.warn(`MoveWarshipExecution: warship ${unitId} is not active`);
+        continue;
+      }
+      warship.setPatrolTile(this.position);
+      warship.setTargetTile(undefined);
     }
-    if (!warship.isActive()) {
-      console.warn("MoveWarshipExecution: warship is not active");
-      return;
-    }
-    warship.setPatrolTile(this.position);
-    warship.setTargetTile(undefined);
   }
 
-  tick(ticks: number): void {}
+  tick(_ticks: number): void {}
 
   isActive(): boolean {
     return false;