fix: compose passes .env through env_file; clearer name and channel messages [CI]

podman-compose does not substitute defaults inside "environment:" (a value
like ${MOSHIRO_NAME:-} reached the node as the literal text), so the node was
enrolled with an invalid name and reported the wrong network scope. The three
compose files now read .env with env_file and only interpolate image and
network_mode; the Backend .env keys are the container's own names
(MoShiro__PublicUrl, MoShiro__ClaimCode).

- The generated .env no longer carries a comment line for MOSHIRO_NAME; the
  dialog says how to choose a name instead.
- The container entrypoint prints the name it enrolls with and defaults
  MOSHIRO_NETWORK_SCOPE to host (Docker Desktop sets container in .env).
- The overview and line pages say "measurement channel not established"
  instead of "not connected": the agent can be reachable and pingable while
  the measurement TCP/UDP path is not.

backend/src/MoShiro.Backend.Application/Admin/JoinTokenService.cs

@@ -134,7 +134,6 @@ public sealed class JoinTokenService
                 lines.Add("MOSHIRO_NETWORK_SCOPE=container");
             }
 
-            lines.Add("# MOSHIRO_NAME=节点名字（不写就用主机名；名字被占用时在这里改）");
             return string.Join('\n', lines);
         }