fix: add missing backslash for volume extraArgs in helm chart (#7676)

Fixes #7467

The -mserver argument line in volume-statefulset.yaml was missing a
trailing backslash, which prevented extraArgs from being passed to
the weed volume process.

Also:
- Extracted master server list generation logic into shared helper
  templates in _helpers.tpl for better maintainability
- Updated all occurrences of deprecated -mserver flag to -master
  across docker-compose files, test files, and documentation

docker/compose/local-dev-compose.yml

@@ -16,7 +16,7 @@ services:
     ports:
       - 8080:8080
       - 18080:18080
-    command: "-v=1 volume -mserver=master:9333 -port=8080 -ip=volume -preStopSeconds=1 -max=10000"
+    command: "-v=1 volume -master=master:9333 -port=8080 -ip=volume -preStopSeconds=1 -max=10000"
     depends_on:
       - master
     volumes: