Serve hashed assets from R2 via CDN_BASE (#3773)

## Description:

Add an optional CDN_BASE env var that prefixes hashed asset URLs from
asset-manifest.json, so the app can serve static assets from R2/CDN
instead of the app origin. The value is determined at runtime via the
EJS template (window.CDN_BASE) — empty string means "same origin,"
matching today's behavior.

A hack to load the worker bundle:

A same-origin Blob script that dynamic-import()s the cross-origin worker
module and buffers early postMessage calls until the imported module's
handler attaches, sidestepping the browser's refusal to construct a
Worker directly from a cross-origin URL.

## 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

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

evan

src/core/worker/WorkerMessages.ts

@@ -28,7 +28,8 @@ export type WorkerMessageType =
   | "attack_clustered_positions"
   | "attack_clustered_positions_result"
   | "transport_ship_spawn"
-  | "transport_ship_spawn_result";
+  | "transport_ship_spawn_result"
+  | "trampoline_error";
 
 // Base interface for all messages
 interface BaseWorkerMessage {
@@ -41,6 +42,7 @@ export interface InitMessage extends BaseWorkerMessage {
   type: "init";
   gameStartInfo: GameStartInfo;
   clientID: ClientID | undefined;
+  cdnBase: string;
 }
 
 export interface TurnMessage extends BaseWorkerMessage {
@@ -137,6 +139,15 @@ export interface TransportShipSpawnResultMessage extends BaseWorkerMessage {
   result: TileRef | false;
 }
 
+// Posted by the Blob trampoline (see WorkerClient.createGameWorker) when the
+// dynamic import of the real worker module fails. The real worker module
+// never loaded, so no other message will ever arrive — initialize() must
+// reject on this rather than wait out its timeout.
+export interface TrampolineErrorMessage extends BaseWorkerMessage {
+  type: "trampoline_error";
+  message: string;
+}
+
 // Union types for type safety
 export type MainThreadMessage =
   | InitMessage
@@ -159,4 +170,5 @@ export type WorkerMessage =
   | PlayerProfileResultMessage
   | PlayerBorderTilesResultMessage
   | AttackClusteredPositionsResultMessage
-  | TransportShipSpawnResultMessage;
+  | TransportShipSpawnResultMessage
+  | TrampolineErrorMessage;