fix(update): target docker-compose.production.yml in dashboard update steps

Production installs use docker-compose.production.yml (the README's documented
path, pinned GHCR images, no dev services), but the dashboard's update
instructions emitted bare `docker compose pull` / `up -d`. Bare `docker compose`
operates on docker-compose.yml — a different, build-based stack — so a
production user who followed the steps:
  - never pulled/recreated their real containers (stayed on the old version,
    e.g. stuck on 3.44.0 after "updating" to 3.45.2), and
  - started the dev-only mailhog service that docker-compose.yml defines
    (reported restart-looping).

The backend runs inside a container and can't stat the host's compose files, but
docker-compose.production.yml passes PICPEAK_RELEASE_CHANNEL into the backend env
and docker-compose.yml does not. detectEnvironment() now derives
isProductionCompose from it, and the Docker update steps prepend
`-f docker-compose.production.yml` when set. The non-production branch keeps the
bare commands but the warning now tells users to add `-f docker-compose.production.yml`
if they installed with it.

Also gates the mailhog service in docker-compose.yml behind a `dev` compose
profile so a plain `docker compose up -d` never starts it (opt in with
`docker compose --profile dev up -d`). Nothing depends on it (SMTP_HOST comes
from .env), so gating is safe. Verified: `docker compose config` lists mailhog
only with `--profile dev`; production compose is unchanged.

Adds unit tests for the production-vs-default command generation.

docker-compose.yml

@@ -141,10 +141,16 @@ services:
     networks:
       - picpeak-network
 
+  # Local mail catcher for development/testing only — never wanted in a real
+  # deployment. Gated behind the `dev` profile so a plain `docker compose up -d`
+  # does NOT start it; opt in with `docker compose --profile dev up -d`. Nothing
+  # depends on it (SMTP_HOST comes from .env), so gating is safe.
   mailhog:
     image: mailhog/mailhog:latest
     container_name: picpeak-mailhog
     restart: unless-stopped
+    profiles:
+      - dev
     ports:
       - "${MAILHOG_SMTP_PORT:-1025}:1025"
       - "${MAILHOG_UI_PORT:-8025}:8025"