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/i18n/locales/de/feed.json

@@ -4,6 +4,7 @@
   "emptyTitle": "Dein Feed ist leer",
   "emptyBody": "Verbinde dein YouTube-Konto, um deine Abos zu importieren und deinen Feed aufzubauen.",
   "setUp": "Meinen Feed einrichten",
+  "browseShared": "Oder einfach die gemeinsame Bibliothek durchsuchen",
   "noMatches": "Keine Videos entsprechen diesen Filtern.",
   "videoCount_one": "{{formattedCount}} Video",
   "videoCount_other": "{{formattedCount}} Videos",

frontend/src/i18n/locales/de/header.json

@@ -3,6 +3,13 @@
   "searchPlaceholder": "Deine Abos durchsuchen…",
   "channelManager": "Kanalverwaltung",
   "usageStats": "Nutzung & Statistik",
+  "scope": {
+    "label": "Feed-Quelle",
+    "my": "Meine",
+    "myTip": "Nur Videos aus deinen eigenen Abos",
+    "all": "Bibliothek",
+    "allTip": "Alle Videos im gemeinsamen Katalog"
+  },
   "account": {
     "admin": "Admin",
     "feed": "Feed",