feat(discovery): total-videos column + subscribe confirmation

Add a "Videos" column (channel total uploads, already in the discovery
response) to the Discover-from-playlists table, and guard the Subscribe button
with a confirm dialog warning it changes the real YouTube account and spends
quota (mirrors the unsubscribe guard). Strings added in EN/HU/DE.

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

@@ -19,7 +19,9 @@
     "subscribedTitle": "Auf YouTube abonniert",
     "subscribedBody": "Du folgst jetzt {{name}} — neue Uploads erscheinen in deinem Feed.",
     "subscribeFailed": "Abonnieren fehlgeschlagen",
+    "confirmSubscribe": "„{{name}}“ auf YouTube abonnieren? Das ändert dein echtes YouTube-Konto und verbraucht etwas API-Kontingent.",
     "cols": {
+      "totalVideos": "Videos",
       "inPlaylists": "In Playlists",
       "inPlaylistsHint": "{{videos}} Video(s) von diesem Kanal in {{playlists}} deiner Playlist(s)."
     }

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

@@ -19,7 +19,9 @@
     "subscribedTitle": "Subscribed on YouTube",
     "subscribedBody": "You're now following {{name}} — its new uploads will start arriving in your feed.",
     "subscribeFailed": "Subscribe failed",
+    "confirmSubscribe": "Subscribe to \"{{name}}\" on YouTube? This changes your real YouTube account and spends a little API quota.",
     "cols": {
+      "totalVideos": "Videos",
       "inPlaylists": "In playlists",
       "inPlaylistsHint": "{{videos}} video(s) from this channel across {{playlists}} of your playlist(s)."
     }

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

@@ -19,7 +19,9 @@
     "subscribedTitle": "Feliratkozva a YouTube-on",
     "subscribedBody": "Mostantól követed: {{name}} — az új feltöltései megjelennek a hírfolyamodban.",
     "subscribeFailed": "A feliratkozás sikertelen",
+    "confirmSubscribe": "Feliratkozol erre a YouTube-on: „{{name}}”? Ez módosítja a valódi YouTube-fiókodat és kevés API-kvótát használ.",
     "cols": {
+      "totalVideos": "Videók",
       "inPlaylists": "Listákban",
       "inPlaylistsHint": "{{videos}} videó ettől a csatornától, {{playlists}} lejátszási listádban."
     }