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/i18n/locales/en/feed.json

@@ -16,6 +16,8 @@
   "demoEmptyBody": "The demo account has no subscriptions of its own — but the whole shared library is yours to explore, filter and sort.",
   "browseLibrary": "Browse the library",
   "noMatches": "No videos match these filters.",
+  "channelScopeHint": "This channel isn't in your subscriptions feed — it may be hidden. Its videos are in the shared library.",
+  "viewInLibrary": "View in Library",
   "videoCount_one": "{{formattedCount}} video",
   "videoCount_other": "{{formattedCount}} videos",
   "sortLabel": "Sort",

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

@@ -16,6 +16,8 @@
   "demoEmptyBody": "A demo fióknak nincsenek saját feliratkozásai — de a teljes közös könyvtár a tiéd: böngészd, szűrd és rendezd kedvedre.",
   "browseLibrary": "Könyvtár böngészése",
   "noMatches": "Egy videó sem felel meg ezeknek a szűrőknek.",
+  "channelScopeHint": "Ez a csatorna nincs a feliratkozásaid feedjében – lehet, hogy el van rejtve. A videói a közös könyvtárban vannak.",
+  "viewInLibrary": "Megnézés a Könyvtárban",
   "videoCount_one": "{{formattedCount}} videó",
   "videoCount_other": "{{formattedCount}} videó",
   "sortLabel": "Rendezés",