feat(feed): Source filter in Mine, Relevance sort, auto search-source

Frontend for the Mine search finds + relevance search:
- The Source filter (organic / include search / search-only) now shows in Mine
  scope too, not just the Library.
- Returning from a YouTube search via 'Back to feed' switches the Source filter
  to 'search' so your just-found videos show in the feed you land on (filtered
  by the kept term).
- New 'Relevance' sort, offered while a search term is present and auto-selected
  when you start searching (reverts to newest when you clear it). EN/HU/DE.

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

@@ -21,7 +21,8 @@
     "title": "Name",
     "subscribers": "Channel subscribers",
     "priority": "Channel priority",
-    "shuffle": "Surprise me"
+    "shuffle": "Surprise me",
+    "relevance": "Relevance"
   },
   "loadingMore": "Loading more…",
   "hiddenNamed": "Hidden “{{title}}”",
@@ -32,7 +33,7 @@
   "unwatch": "Unwatch",
   "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.",
+    "tip": "Filter by how videos got here: your subscriptions/catalog only, plus videos you found via live YouTube search, or only those search results.",
     "organic": "Hide search results",
     "all": "Include search results",
     "only": "Search results only"