chore(ui): dedup last 3 jscpd clones → 0 (collapsed rail, video-card text block, subs column)

- CollapsedFilterRail: the byte-identical 31-line collapsed sidebar rail was in
  both Sidebar (feed) and PlexSidebar → one shared component.
- VideoCard: the title + channel-button + meta block was duplicated across the
  list-row and card layouts → a local `textBlock` element (only one layout branch
  renders, so reusing the element is safe).
- subsColumn<T>(t): the identical 'subscribers' DataTable column in Channels +
  ChannelDiscovery → a generic factory in channelColumns.tsx.

All behavior-neutral; jscpd now reports 0 clones (was 8 at Phase 0 baseline).
tsc + knip green.

frontend/src/components/ChannelDiscovery.tsx

@@ -4,6 +4,7 @@ import { UserPlus } from "lucide-react";
 import { api, type DiscoveredChannel } from "../lib/api";
 import { accountKey, LS } from "../lib/storage";
 import { formatCountOrDash } from "../lib/format";
+import { subsColumn } from "./channelColumns";
 import { notify } from "../lib/notifications";
 import { notifyYouTubeActionError } from "../lib/youtubeErrors";
 import Tooltip from "./Tooltip";
@@ -78,17 +79,7 @@ export default function ChannelDiscovery({
         <ChannelLink id={c.id} title={c.title} handle={c.handle} thumbnailUrl={c.thumbnail_url} />
       ),
     },
-    {
-      key: "subs",
-      header: t("channels.cols.subs"),
-      align: "right",
-      nowrap: true,
-      sortable: true,
-      sortValue: (c) => c.subscriber_count ?? -1,
-      render: (c) => (
-        <span className="text-muted tabular-nums">{formatCountOrDash(c.subscriber_count)}</span>
-      ),
-    },
+    subsColumn<DiscoveredChannel>(t),
     {
       key: "videos",
       header: t("channels.discovery.cols.totalVideos"),