New SubscriptionsStore (SharedPrefs-lite, same pattern as History/Settings).
Holds Set<ChannelRef> { url, name, avatar }.
ChannelScreen header now has a "Subscribe / Subscribed" button on the
right. Button toggles membership; UI updates immediately via StateFlow.
StrawHome: new "Subscriptions" section above "Recently watched", a
LazyRow of subscribed channels (avatar + name, 80dp chips). Tap a
subscription chip → opens that channel.
StrawApp.onCreate: Subscriptions.init(this).
Day-4 ideas: auto-aggregate latest videos from subs on Home (no more
"hit search to see videos"), sub-feed-only screen, channel notifications.
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