fix(channel): hide the Source (search-results) filter on the channel-scoped feed

The Source selector (organic / include-search / search-only) is a global-catalog
concept — it picks how videos ENTERED the library. On a single channel's page the
view is pinned to librarySource=all and would only offer a near-empty, confusing
slice, so hide the selector there (the main feed keeps it).

frontend/src/components/Feed.tsx

@@ -552,6 +552,12 @@ export default function Feed({
             </button>
           );
         })}
+        {/* The Source filter (organic / include-search / search-only) is a global-catalog concept —
+            it selects how videos ENTERED the library. On a channel-scoped view we already show all
+            of this one channel's videos (librarySource is pinned to "all"), so the selector would
+            only offer a near-empty, confusing slice — hide it here. */}
+        {!channelScoped && (
+          <>
             <span className="mx-1 h-5 w-px bg-border" aria-hidden="true" />
             <label className="inline-flex items-center gap-1.5 text-xs text-muted">
               <span>{t("feed.source.label")}</span>
@@ -571,6 +577,8 @@ export default function Feed({
                 <option value="search">{t("feed.source.only")}</option>
               </select>
             </label>
+          </>
+        )}
         <span className="mx-1 h-5 w-px bg-border" aria-hidden="true" />
         <span className="text-xs text-muted whitespace-nowrap">
           {countQuery.data