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/alembic/versions/0055_channel_keywords.py

@@ -0,0 +1,23 @@
+"""channels.keywords: creator keyword tags from brandingSettings (About tab)
+
+Revision ID: 0055_channel_keywords
+Revises: 0054_audit_log
+Create Date: 2026-07-12
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = "0055_channel_keywords"
+down_revision: Union[str, None] = "0054_audit_log"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column("channels", sa.Column("keywords", sa.Text(), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column("channels", "keywords")