feat(channels): YouTube link on channel name + done checkmarks

- Channel name: middle-click opens the channel's YouTube page in a new tab
  (left-click still opens the in-app detail) plus an explicit external-link icon.
- recent/full sync badges show a check icon when the state is reached.
- New i18n key channels.row.openOnYouTube (HU/EN/DE).

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

@@ -36,6 +36,7 @@
   "row": {
     "stored": "{{formatted}} stored",
     "subs": "{{formatted}} subs",
+    "openOnYouTube": "Open on YouTube",
     "priorityHint": "Your ranking for this channel. Sort the feed by “Channel priority” to bring higher-priority channels to the top.",
     "raisePriority": "Raise priority",
     "lowerPriority": "Lower priority",