fix: don't overwrite Node.channel from MAP_REPORT_APP if already set (#147)

NODEINFO_APP is the better / authoritative source for a nodes primary channel.
MAP_REPORT_APP packets can arrive on a different MQTT uplink channel and
are unconditionally overwriting the channel set by NODEINFO, causing
nodes to appear on the wrong channel in the map and channel filters.

now MAP_REPORT_APP only sets Node.channel when it is not (yetw) known,
making it a fallback rather than an override.

meshview/mqtt_store.py

@@ -109,7 +109,8 @@ async def process_envelope(topic, env):
                     node.short_name = map_report.short_name
                     node.hw_model = hw_model
                     node.role = role
-                    node.channel = env.channel_id
+                    if not node.channel:
+                        node.channel = env.channel_id
                     node.last_lat = map_report.latitude_i
                     node.last_long = map_report.longitude_i
                     node.firmware = map_report.firmware_version