refactor(docker): make image-based the default compose, build variant is .dev

Less confusion: docker-compose.yml is now the default (pulls pre-built images)
and the build-on-host variant is docker-compose.dev.yml. Updated all references
(CLAUDE.md, README, .env.example, build-push-images.mjs, compose header).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.env.example

@@ -1,5 +1,5 @@
 # Copy to .env next to your docker-compose file and fill in.  cp .env.example .env
-# Used by docker-compose.yml (build-on-host) AND docker-compose.prod.yml (pulls images).
+# Used by docker-compose.yml (pulls pre-built images) AND docker-compose.dev.yml (build-on-host).
 
 # ── URLs pre-filled on the frontend setup screen ─────────────────────────────
 NAVIDROME_URL=https://music.example.com
@@ -22,7 +22,7 @@ GITEA_REPO=Docker/NovaSonic
 GITEA_TOKEN=                              # read token (only needed for a private repo)
 GITEA_APK_TAG=apk-latest
 
-# ── Pre-built images (docker-compose.prod.yml only) ──────────────────────────
+# ── Pre-built images (docker-compose.yml only) ───────────────────────────────
 # REGISTRY = where `npm run push-images` pushed to and where the host pulls from.
 #   Gitea container registry (lowercase owner):  gitea.honeyy.duckdns.org/docker
 #   Docker Hub:                                  docker.io/youruser