Add trade ship captured event with toggle setting (#4344)

## What

Notify a player when one of their trade ships is captured. The alert
appears in the **less-important (top) events tier** and is gated behind
a new in-game setting (on by default).

## Why

Previously there was no notification to the player who *lost* a trade
ship — only the capturer got a transient +gold pip on the ship's
arrival. This surfaces the loss to the victim, while letting players opt
out if they find it noisy.

## Changes

- **`src/core/execution/TradeShipExecution.ts`** — On capture detection,
emit a display message (`events_display.trade_ship_captured`, type
`UNIT_DESTROYED`) to the original owner. Fires once, guarded by the
existing `wasCaptured` flag. `UNIT_DESTROYED` is not a Tier-1 type, so
it lands in the top/less-important tier.
- **`src/client/hud/layers/EventsDisplay.ts`** — Suppress the message
when the setting is off, following the existing key-based filter
pattern.
- **`src/core/game/UserSettings.ts`** — New `tradeShipCapturedEvents()`
getter (default `true`) + `toggleTradeShipCapturedEvents()`.
- **`src/client/hud/layers/SettingsModal.ts`** — New toggle in the
in-game settings modal.
- **`resources/lang/en.json`** — New
`events_display.trade_ship_captured` and
`user_setting.trade_ship_captured_label`/`_desc` keys.
- **`tests/core/executions/TradeShipExecution.test.ts`** — Tests that
the notification is sent to the original owner with the right args and
only once across ticks.

## Notes

- The setting is gated client-side (in `EventsDisplay`), keeping
`src/core` free of client-local localStorage settings — consistent with
how display events are already filtered there.
- Reused `MessageType.UNIT_DESTROYED` (red/"loss" styling) rather than
adding a new message type, to keep the change minimal. Happy to add a
dedicated type/color if preferred.

## Testing

- `npx vitest tests/core/executions/TradeShipExecution.test.ts --run` —
7 passed
- lint clean, no type errors

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

resources/lang/en.json

@@ -423,6 +423,7 @@
     "sent_emoji": "Sent {name}: {emoji}",
     "sent_gold_to_player": "Sent {gold} gold to {name}",
     "sent_troops_to_player": "Sent {troops} troops to {name}",
+    "trade_ship_captured": "Your trade ship was captured by {name}",
     "unit_destroyed": "Your {unit} was destroyed",
     "unit_voluntarily_deleted": "Unit voluntarily deleted",
     "wants_to_renew_alliance": "{name} wants to renew your alliance"

src/core/execution/TradeShipExecution.ts

@@ -69,6 +69,14 @@ export class TradeShipExecution implements Execution {
     if (this.wasCaptured !== true && this.origOwner !== tradeShipOwner) {
       // Store as variable in case ship is recaptured by previous owner
       this.wasCaptured = true;
+      this.mg.displayMessage(
+        "events_display.trade_ship_captured",
+        MessageType.UNIT_DESTROYED,
+        this.origOwner.id(),
+        undefined,
+        { name: tradeShipOwner.displayName() },
+        this.tradeShip.id(),
+      );
     }
 
     // If a player captures another player's port while trading we should delete

tests/core/executions/TradeShipExecution.test.ts

@@ -1,5 +1,5 @@
 import { TradeShipExecution } from "../../../src/core/execution/TradeShipExecution";
-import { Game, Player, Unit } from "../../../src/core/game/Game";
+import { Game, MessageType, Player, Unit } from "../../../src/core/game/Game";
 import { PathStatus } from "../../../src/core/pathfinding/types";
 import { setup } from "../../util/Setup";
 
@@ -135,6 +135,26 @@ describe("TradeShipExecution", () => {
     expect(tradeShip.setTargetUnit).toHaveBeenCalledWith(piratePort);
   });
 
+  it("should notify the original owner when the ship is captured", () => {
+    tradeShip.owner = vi.fn(() => pirate);
+    tradeShipExecution.tick(1);
+    expect(game.displayMessage).toHaveBeenCalledWith(
+      "events_display.trade_ship_captured",
+      MessageType.UNIT_DESTROYED,
+      origOwner.id(),
+      undefined,
+      { name: pirate.displayName() },
+      tradeShip.id(),
+    );
+  });
+
+  it("should only notify the original owner once across ticks", () => {
+    tradeShip.owner = vi.fn(() => pirate);
+    tradeShipExecution.tick(1);
+    tradeShipExecution.tick(2);
+    expect(game.displayMessage).toHaveBeenCalledTimes(1);
+  });
+
   it("should complete trade and award gold", () => {
     tradeShipExecution["pathFinder"] = {
       next: vi.fn(() => ({ status: PathStatus.COMPLETE, node: 32 })),