feat(design): a channel's and a Plex playlist's one action list (C1) — the row's "…", Delete + undo, an action toast's 8 s floor (E28 S3e2)

- Channels: the row's full-history, hide and unsubscribe buttons become its "…" at the row end
  (useChannelActions: open, pin, YouTube, copy name / link, full history or the admin reset,
  hide with an undo toast, unsubscribe behind its danger confirm); the card layout docks the "…"
  beside the name (ChannelLayoutGrid moreKey). The admin reset's confirm now names the reset.
- Plex playlist page: a title, a show group and a season each open the page's one row menu (a
  title's is its usePlexCardActions list plus the removal); no X at rest; removals take an undo
  toast whose Undo puts every row back at its own index in one request (bulk add `positions`);
  j/k walk the visible rows, Delete removes the walked one; rename and delete in the header's "…".
- notify: a toast with an action lives at least 8 s (the owner's call), whatever the toast setting.

frontend/src/components/channelColumns.dom.test.tsx

@@ -5,7 +5,7 @@
 // "fully" | "needsFull" | "" vocab + predicate — asserted here so a regression can't silently change
 // which channels the top chips show.
 import { describe, it, expect } from "vitest";
-import { applySyncFilter, tagsColumnTest } from "./channelColumns";
+import { applySyncFilter, buildChannelColumns, tagsColumnTest } from "./channelColumns";
 import { NO_TAG_SENTINEL, type ManagedChannel } from "../lib/api";
 
 const ch = (over: Partial<ManagedChannel>): ManagedChannel => ({
@@ -71,3 +71,28 @@ describe("tagsColumnTest (item B — 'No tag')", () => {
     expect(tagsColumnTest(tagged, [NO_TAG_SENTINEL, "1"])).toBe(false);
   });
 });
+
+describe("buildChannelColumns — the row's end", () => {
+  it('ends every row in the channel\'s "…", with no action buttons before it', () => {
+    const more = (c: ManagedChannel) => `more:${c.id}`;
+    const noop = () => {};
+    const cols = buildChannelColumns({
+      t: ((k: string) => k) as never,
+      prioOptions: [],
+      userTags: [],
+      bumpPriority: noop,
+      setPriority: noop,
+      more,
+      onView: noop,
+      toggleTag: noop,
+      createAndAttachTag: noop,
+      onFilterByTag: noop,
+      suggestionsFor: () => [],
+      onAcceptSuggestion: noop,
+      onDismissSuggestion: noop,
+    });
+    expect(cols[cols.length - 1]!.key).toBe("actions");
+    expect(cols.filter((c) => c.key === "actions")).toHaveLength(1);
+    expect(cols[cols.length - 1]!.render(ch({ id: "UCz" }))).toBe("more:UCz");
+  });
+});