feat(managers): paginate the card layout + its top scroll-fade

UAT round 3 feedback on the card layout:
- Add a pager + "Cards per page" to the card view (it infinitely-scrolled before).
  Extracted the pager cluster out of DataTable into a reusable Pager component
  (the table now renders it too — no behaviour change); the card view slices the
  sorted rows and drives its own per-account page size (subscribed + discovery).
- Restore the top edge-fade in the card layout. The channels page registers
  fadeTop:false for its sticky-header table; the card layout has no sticky header,
  so App now re-enables the top fade when the channel layout is cards.

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

@@ -19,6 +19,7 @@
     "asc": "Ascending",
     "desc": "Descending"
   },
+  "cardsPerPage": "Cards per page",
   "discovery": {
     "intro": "Channels that appear in your playlists but that you don't subscribe to. Subscribe to follow them — their new uploads will start showing in your feed (subscribing costs a little YouTube quota; existing videos aren't re-fetched).",
     "loading": "Finding channels…",

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

@@ -19,6 +19,7 @@
     "asc": "Növekvő",
     "desc": "Csökkenő"
   },
+  "cardsPerPage": "Kártya / oldal",
   "discovery": {
     "intro": "Csatornák, amelyek megjelennek a lejátszási listáidban, de még nem iratkoztál fel rájuk. Iratkozz fel, hogy kövesd őket — az új feltöltéseik megjelennek a hírfolyamodban (a feliratkozás kevés YouTube-kvótát használ; a meglévő videókat nem tölti le újra).",
     "loading": "Csatornák keresése…",