Add a "Mine / Library" segmented toggle (shown on the feed page) that switches
FeedFilters.scope between the user's own subscriptions and the whole shared
catalog. A read-scope-less user (signed in but no YouTube grant) can now browse
and manage the shared library on their own account — the empty "my feed" state
offers a "browse the shared library" shortcut alongside the connect-YouTube CTA.
scope is preserved across "Clear all" (it's a mode, not a filter) and kept out
of the URL state. Trilingual strings (HU/EN/DE) for the toggle and the CTA.
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