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/channelColumns.tsx

@@ -0,0 +1,18 @@
+import type { TFunction } from "i18next";
+import { formatCountOrDash } from "../lib/format";
+import type { Column } from "./DataTable";
+
+/** The shared "subscribers" DataTable column (right-aligned, sortable, em-dash for null) — identical
+ * in the Channels manager and the ChannelDiscovery table, which show subscriber counts on different
+ * channel row shapes. */
+export function subsColumn<T extends { subscriber_count: number | null }>(t: TFunction): Column<T> {
+  return {
+    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>,
+  };
+}