feat(channel): enrich the About tab + fix the tab-switch header shift

- fix: reserve the scrollbar gutter (scrollbar-gutter:stable) on the channel
  page's scroll container, so switching between the tall Videos tab and the
  short About tab no longer shifts the banner/avatar/buttons a few px sideways
  (the vertical scrollbar was appearing/disappearing between the two tabs).
- About tab now shows Country (flag + localized name), Language, Topics
  (topicCategories → readable chips), and Keywords (brandingSettings keywords
  parsed into chips, quoted multi-word tags kept whole). country/language/topics
  were already stored; keywords is new (migration 0055_channel_keywords, mapped
  in apply_channel_details, returned by channel_detail).
- Discovery → channel page: the Channel-manager "Discover from playlists" tab now
  links each channel name to its in-app channel page (ChannelLink onView), so a
  discovered channel's About/videos can be inspected BEFORE subscribing (was
  subscribe-only, plain text before).

Note: the channel info-card epic itself was already delivered in v0.19.0; this is
the About-tab enrichment follow-up + the header-shift fix. external_links stays
empty by design (YouTube removed the field from the Data API ~2023).

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

@@ -19,5 +19,9 @@
   "noDescription": "This channel has no description.",
   "block": "Block channel",
   "unblock": "Unblock channel",
-  "blockedBadge": "Blocked"
+  "blockedBadge": "Blocked",
+  "country": "Country",
+  "language": "Language",
+  "topics": "Topics",
+  "keywords": "Keywords"
 }