feat(feed): guide a hidden-channel empty result to the Library

Filtering by a channel under the Mine scope returns nothing when that channel's
subscription is hidden (or you don't subscribe to it) — its videos only live in
the shared Library, since the Mine feed excludes hidden subscriptions by design.
Previously this left a bare 'No videos match these filters', which reads as a
bug when you know you're subscribed.

Add an empty-state hint + a 'View in Library' button (switches scope to all) when
there's a channel filter under Mine and zero results. Bilingual EN/HU.

Verified live: ?channel=<ColdFusion>&show=all under Mine shows the hint; the
button switches to Library and surfaces the 36 videos.

frontend/src/components/Feed.tsx

@@ -13,6 +13,7 @@ import {
   Grid3x3,
   Info,
   LayoutGrid,
+  Library,
   List,
   RefreshCw,
   Rows3,
@@ -686,6 +687,21 @@ export default function Feed({
       {items.length === 0 ? (
         <div className="py-16 text-center text-muted">
           <p>{t("feed.noMatches")}</p>
+          {/* A channel filter under the "Mine" scope returns nothing when that channel is hidden from
+              your subscriptions (or you don't subscribe to it) — its videos only live in the shared
+              Library. Point there rather than leaving a bare "no matches". */}
+          {filters.channelIds.length > 0 && filters.scope === "my" && (
+            <div className="mt-4">
+              <p className="text-sm">{t("feed.channelScopeHint")}</p>
+              <button
+                onClick={() => setFilters({ ...filters, scope: "all" })}
+                className="mt-3 inline-flex items-center gap-2 px-4 py-2 rounded-xl font-semibold btn-primary transition"
+              >
+                <Library className="w-4 h-4" />
+                {t("feed.viewInLibrary")}
+              </button>
+            </div>
+          )}
           {filters.q.trim() && !isDemo && (
             <button
               onClick={() => onYtSearch(filters.q.trim())}