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).

backend/app/models.py

@@ -180,6 +180,9 @@ class Channel(Base, TimestampMixin):
     topic_categories: Mapped[list | None] = mapped_column(JSON)
     default_language: Mapped[str | None] = mapped_column(String(16))
     country: Mapped[str | None] = mapped_column(String(8))
+    # Creator keyword tags (brandingSettings.channel.keywords) — a single space-separated string,
+    # multi-word tags quoted; the client parses it into chips for the About tab.
+    keywords: Mapped[str | None] = mapped_column(Text)
 
     # Sync bookkeeping.
     details_synced_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))

backend/app/routes/channels.py

@@ -233,6 +233,9 @@ def _channel_detail_dict(
         "published_at": channel.published_at.isoformat() if channel.published_at else None,
         "external_links": channel.external_links or [],
         "country": channel.country,
+        "default_language": channel.default_language,
+        "topic_categories": channel.topic_categories or [],
+        "keywords": channel.keywords,
         "subscribed": subscribed,
         "explored": explored,
         "blocked": blocked,

backend/app/sync/subscriptions.py

@@ -69,6 +69,7 @@ def apply_channel_details(db: Session, items: list[dict]) -> None:
         channel.banner_url = branding.get("image", {}).get("bannerExternalUrl")
         channel.external_links = _external_links(branding)
         channel.topic_categories = topics.get("topicCategories")
+        channel.keywords = branding.get("channel", {}).get("keywords")
         channel.details_synced_at = now