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

tests/Warship.test.ts

@@ -164,7 +164,11 @@ describe("Warship", () => {
     game.addExecution(new WarshipExecution(warship));
 
     game.addExecution(
-      new MoveWarshipExecution(player1, warship.id(), game.ref(coastX + 5, 15)),
+      new MoveWarshipExecution(
+        player1,
+        [warship.id()],
+        game.ref(coastX + 5, 15),
+      ),
     );
 
     executeTicks(game, 10);
@@ -244,7 +248,7 @@ describe("Warship", () => {
     );
     new MoveWarshipExecution(
       player2,
-      warship.id(),
+      [warship.id()],
       game.ref(coastX + 5, 15),
     ).init(game, 0);
     expect(warship.patrolTile()).toBe(originalPatrolTile);
@@ -262,7 +266,7 @@ describe("Warship", () => {
     warship.delete();
     new MoveWarshipExecution(
       player1,
-      warship.id(),
+      [warship.id()],
       game.ref(coastX + 5, 15),
     ).init(game, 0);
     expect(warship.patrolTile()).toBe(originalPatrolTile);
@@ -271,7 +275,7 @@ describe("Warship", () => {
   test("MoveWarshipExecution fails gracefully if warship not found", async () => {
     const exec = new MoveWarshipExecution(
       player1,
-      123,
+      [123],
       game.ref(coastX + 5, 15),
     );