feat(search): Library provenance filter (organic / all / search-only)

Replace the binary 'show search-discovered' toggle with a 3-way Source selector in the
Library toolbar, so users can also see ONLY search-discovered videos — not just hide or
mix them. Backend: feed param library_source = organic (default, hides via_search) | all
(both) | search (only via_search), applied in scope=all. Strings in HU/EN/DE.

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

@@ -30,8 +30,13 @@
   "markedWatchedNamed": "Marked watched “{{title}}”",
   "markedWatched": "Marked watched",
   "unwatch": "Unwatch",
-  "searchDiscovered": "Search-discovered",
-  "searchDiscoveredTip": "Also show videos that entered the library only through a live YouTube search.",
+  "source": {
+    "label": "Source",
+    "tip": "Filter the Library by how videos got here: organic catalog only, plus search results, or only videos found via live YouTube search.",
+    "organic": "Hide search results",
+    "all": "Include search results",
+    "only": "Search results only"
+  },
   "yt": {
     "searchFor": "Search YouTube for “{{query}}”",
     "resultsFor": "YouTube results for “{{query}}”",