fix(feed): reliably switch to relevance sort when a search starts

The relevance auto-select ran in a Feed effect that raced per-keystroke query
updates, so fast typing left the sort on the default. Move the switch into the
header input's onChange so it's set atomically with the query (only overriding
the default newest sort); the Feed effect now only reverts to newest when the
term is cleared. 'Back to feed' also sets relevance explicitly.

frontend/src/components/Feed.tsx

@@ -137,19 +137,15 @@ export default function Feed({
     retry: false,
   });
 
-  // When a search term first appears, rank the local feed by relevance (YouTube-like); when
-  // it's cleared, fall back to the default newest sort. Only fires on the empty↔non-empty
-  // transition, so it never overrides a sort the user picks while a query stays active.
+  // Switching to the relevance sort when a search starts happens atomically in the header's
+  // input onChange (race-free with the query update). Here we only handle the reverse: when
+  // the term is cleared, fall back to the default sort — relevance has no dropdown option and
+  // doesn't rank without a query.
   const qEmpty = !filters.q.trim();
-  const prevQEmptyRef = useRef(qEmpty);
   useEffect(() => {
-    const sortKeyNow = parseSort(filters.sort).key;
-    if (prevQEmptyRef.current && !qEmpty && sortKeyNow !== "relevance") {
-      setFilters({ ...filters, sort: "relevance" });
-    } else if (!prevQEmptyRef.current && qEmpty && sortKeyNow === "relevance") {
+    if (qEmpty && parseSort(filters.sort).key === "relevance") {
       setFilters({ ...filters, sort: buildSort("date", "desc") });
     }
-    prevQEmptyRef.current = qEmpty;
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [qEmpty]);
 
@@ -316,8 +312,8 @@ export default function Feed({
               // disabled and still holds its pre-search cache) is stale. Drop those caches so
               // it re-fetches fresh on return instead of showing the old (often empty) result.
               // Surface the just-searched videos in the feed you return to: switch the Source
-              // filter to "search" so your own search finds show (filtered by the kept term).
-              setFilters({ ...filters, librarySource: "search" });
+              // filter to "search" (your own search finds) and rank them by relevance.
+              setFilters({ ...filters, librarySource: "search", sort: "relevance" });
               onExitYtSearch();
               qc.removeQueries({ queryKey: ["feed"] });
               qc.removeQueries({ queryKey: ["feed-count"] });