feat(managers): richer channel cards + drop the rows layout, cap the channel column

UAT round 1 feedback:
- Retire the `rows` layout — it barely differed from the table; keep Table + Cards.
- The card now shows the About cell (+ its hover overlay) and the priority stepper
  in every layout, not just the table: ChannelLayoutGrid gains named slots
  (lead / about / actions) and stops hiding the `hideInCard` columns, since the
  roomy desktop card has space the cramped mobile fallback did not.
- Fix the table losing its centering with "Rows per page: All": a long-titled
  channel stretched the uncapped Channel column (~289→429px) and overflowed the
  centered table. Cap it with a max-w element inside the cell (table-layout:auto
  ignores a cell max-width), the same trick AboutCell already uses. Gaps now 163/162.

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

@@ -11,8 +11,7 @@
   "layoutLabel": "Layout",
   "layout": {
     "table": "Table",
-    "cards": "Cards",
-    "rows": "Rows"
+    "cards": "Cards"
   },
   "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).",

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

@@ -11,8 +11,7 @@
   "layoutLabel": "Nézet",
   "layout": {
     "table": "Táblázat",
-    "cards": "Kártya",
-    "rows": "Sor"
+    "cards": "Kártya"
   },
   "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).",