Deploy: make a missing .env name its own fix

With no .env found, compose fell back to a sentinel path and the daemon
reported: Bind mount failed: 'DATA_ROOT-is-unset/postgres' does not exist —
accurate, but it tells the operator nothing about what to do, and this is the
most common way the install goes wrong (the file is generated by `init`, not
shipped in the repo, so any re-copy of the folder loses it).

The sentinel is now /NO-.env-FOUND-RUN-deploy_webmail.sh-init-FIRST, so the
daemon's own error states the fix. doctor also checks for a missing or empty
.env before anything else and says the same thing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Nux3LCu4zP9nih4BRhPyHw

deploy/webmail.sh

@@ -234,6 +234,10 @@ cmd_doctor() {
     require_env
     local ok=0
 
+    if [ ! -s "$ENV_FILE" ]; then
+        red "  $ENV_FILE is missing or empty — nothing will start. Run: webmail.sh init" >&2
+    fi
+
     echo "config:"
     echo "  DATA_ROOT      ${DATA_ROOT}"
     echo "  APP_DOMAIN     ${APP_DOMAIN:-<unset>}"