Fix postMessage type error in Web Worker files by adding triple-slash directives (#16761)

* Initial plan

* Add triple-slash directives to worker files to fix postMessage type error

Co-authored-by: syuilo <4439005+syuilo@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: syuilo <4439005+syuilo@users.noreply.github.com>

packages/frontend-embed/src/workers/draw-blurhash.ts

@@ -3,6 +3,9 @@
  * SPDX-License-Identifier: AGPL-3.0-only
  */
 
+/// <reference lib="esnext" />
+/// <reference lib="webworker" />
+
 import { render } from 'buraha';
 
 const canvas = new OffscreenCanvas(64, 64);

packages/frontend-embed/src/workers/test-webgl2.ts

@@ -3,6 +3,9 @@
  * SPDX-License-Identifier: AGPL-3.0-only
  */
 
+/// <reference lib="esnext" />
+/// <reference lib="webworker" />
+
 const canvas = globalThis.OffscreenCanvas && new OffscreenCanvas(1, 1);
 // 環境によってはOffscreenCanvasが存在しないため
 // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition

packages/frontend/src/workers/draw-blurhash.ts

@@ -3,6 +3,9 @@
  * SPDX-License-Identifier: AGPL-3.0-only
  */
 
+/// <reference lib="esnext" />
+/// <reference lib="webworker" />
+
 import { render } from 'buraha';
 
 const canvas = new OffscreenCanvas(64, 64);

packages/frontend/src/workers/test-webgl2.ts

@@ -3,6 +3,9 @@
  * SPDX-License-Identifier: AGPL-3.0-only
  */
 
+/// <reference lib="esnext" />
+/// <reference lib="webworker" />
+
 const canvas = globalThis.OffscreenCanvas && new OffscreenCanvas(1, 1);
 // 環境によってはOffscreenCanvasが存在しないため
 // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition