feat(subscriptions): add subscription to group from channel context menu

[guthubrx]

Summary

• Adds an "Add to group" sub-menu in the subscription context menu on the channel page
• Shows all existing subscription groups with a checkmark for groups the channel already belongs to
• Toggling a group immediately saves the updated membership via subscriptionGroupSave()
• Groups are fetched in parallel with subscription settings via Promise.all (no latency regression)
• Graceful fallback: if group fetch fails, the menu still opens without the "Add to group" section

Files changed

• src/Menus.tsx — getSubscriptionMenu() accepts groups parameter, adds MenuItemGroup sub-menu with toggle behavior
• src/pages/Channel/index.tsx — parallel fetch with Promise.all + .catch() fallback, passes groups to menu builder

[futo-cla-pr-labler]

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[futo-cla-pr-labler]

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[guthubrx]

/recheck