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/en/feed.json

@@ -4,6 +4,7 @@
   "emptyTitle": "Your feed is empty",
   "emptyBody": "Connect your YouTube account to import your subscriptions and build your feed.",
   "setUp": "Set up my feed",
+  "browseShared": "Or just browse the shared library",
   "noMatches": "No videos match these filters.",
   "videoCount_one": "{{formattedCount}} video",
   "videoCount_other": "{{formattedCount}} videos",

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

@@ -3,6 +3,13 @@
   "searchPlaceholder": "Search your subscriptions…",
   "channelManager": "Channel manager",
   "usageStats": "Usage & stats",
+  "scope": {
+    "label": "Feed source",
+    "my": "Mine",
+    "myTip": "Only videos from your own subscriptions",
+    "all": "Library",
+    "allTip": "Every video in the shared catalog"
+  },
   "account": {
     "admin": "Admin",
     "feed": "Feed",