Fix sync group toggle (#3142)

* Fix dynamic sync group toggle having no effect on member checkboxes

PR #3118 made _attr_static_group_members unconditionally populated for
both dynamic and static groups. Since the frontend uses
static_group_members to determine which players are non-removable
(greyed out, no checkbox), this caused all members to appear locked
regardless of the dynamic toggle setting.

Restore the conditional assignment: dynamic groups get an empty
static_group_members list (all members removable), static groups get
the configured members list (all members locked). Fix the power-off
reset to read configured members directly from config instead of
_attr_static_group_members, so dynamic groups preserve their member
list on power-off without needing them marked as static.

https://claude.ai/code/session_01VVeBMzkvqTrutpg53aMtBY

* Remove added comments

https://claude.ai/code/session_01VVeBMzkvqTrutpg53aMtBY

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Co-authored-by: Claude <[email protected]>