Pin the frontend image to v1.3.0

The server image used musicfrontend:latest, so it got whatever frontend was
newest when it was built. Pin the version instead, and bump it when a new
frontend is released.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Dockerfile

@@ -1,5 +1,5 @@
-# The frontend is built and published separately; override to pin a version.
-ARG FRONTEND_IMAGE=gitea.sanplex.xyz/sansan/musicfrontend:latest
+# The frontend is built and published separately. Bump this when a new frontend is released.
+ARG FRONTEND_IMAGE=gitea.sanplex.xyz/sansan/musicfrontend:v1.3.0
 
 # Stage 1: Build the backend on the build machine and cross-compile for the target platform,
 # so multi-platform builds need no emulation.