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/client/HelpModal.ts

@@ -434,6 +434,28 @@ export class HelpModal extends BaseModal {
                       ${translateText("help_modal.action_auto_upgrade")}
                     </td>
                   </tr>
+                  <tr class="hover:bg-white/5 transition-colors">
+                    <td class="py-3 pl-4 border-b border-white/5">
+                      <div class="inline-flex items-center gap-2">
+                        ${this.renderKey(keybinds.shiftKey)}
+                        <span class="text-white/40 font-bold">+</span>
+                        <span class="text-white/50 text-xs"
+                          >${translateText("help_modal.drag")}</span
+                        >
+                      </div>
+                    </td>
+                    <td class="py-3 border-b border-white/5 text-white/70">
+                      ${translateText("help_modal.action_warship_multiselect")}
+                    </td>
+                  </tr>
+                  <tr class="hover:bg-white/5 transition-colors">
+                    <td class="py-3 pl-4 border-b border-white/5">
+                      ${this.renderKey(keybinds.selectAllWarships)}
+                    </td>
+                    <td class="py-3 border-b border-white/5 text-white/70">
+                      ${translateText("help_modal.action_warship_selectall")}
+                    </td>
+                  </tr>
                 </tbody>
               </table>
             </div>