feat(feed): shared-library scope toggle in the header

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.

frontend/src/App.tsx

@@ -39,6 +39,7 @@ const DEFAULT_FILTERS: FeedFilters = {
   tagMode: "or",
   q: "",
   sort: "newest",
+  scope: "my",
   includeNormal: true,
   includeShorts: false,
   includeLive: false,