Enable ARM64 support in CI build workflow

Add linux/arm64 to the platforms list for multi-architecture container builds.

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

Co-Authored-By: Claude <[email protected]>

Author: csenet

.github/workflows/image-build.yml

@@ -31,7 +31,7 @@ jobs:
         with:
           context: ./app
           file: ./app/Dockerfile
-          platforms: linux/amd64
+          platforms: linux/amd64,linux/arm64
           push: true
           tags: ghcr.io/102ch/discord-tts-bot:${{ env.IMAGE_TAG }},ghcr.io/102ch/discord-tts-bot:latest
           cache-from: type=gha