fix(ui): focus, paging and state that a key or a finger can reach

- A key landing on a side panel's tab steps into the card it peeks open, not onto the hidden tab.
- The Channels cards keep their status / pager row at the list's end while the band is folded.
- A menu row's note shows on an enabled row too: the download state under 'Download…', not a title.
- A removal chip hands the focus to its neighbour before it goes, for every caller (the feed's and
  Plex's active filters); the hidden-filter cue's own copy is gone.
- Back to a spot within a page's end spacer lands at the page's end, not its top.
- A running push / revert is tracked per list: another list's push is no longer refused unseen.

frontend/src/components/ui/Chip.dom.test.tsx

@@ -3,7 +3,8 @@
 // E28 S3b: the chip family (docs/DESIGN.md "One primitive per button family"). One geometry for a
 // filter, an applied narrowing, a tag and a suggestion, where five files had five.
 import { afterEach, describe, expect, it, vi } from "vitest";
-import { cleanup, fireEvent, render, screen } from "@testing-library/react";
+import { act, cleanup, fireEvent, render, screen } from "@testing-library/react";
+import { useState } from "react";
 import { Chip, DismissibleChip } from "./Chip";
 import { chipClass } from "./chipStyles";
 
@@ -41,6 +42,34 @@ describe("Chip", () => {
     expect(onClick).toHaveBeenCalledOnce();
   });
 
+  // Round 2 (F6): the feed's and Plex's active-filter chips unmount with the filter they remove,
+  // and a keyboard press left the focus on <body>. The chip hands it on itself, for every caller.
+  it("a removing chip pressed from the keyboard hands the focus to its neighbour", () => {
+    function Row() {
+      const [chips, setChips] = useState(["Music", "News"]);
+      return (
+        <div>
+          {chips.map((c) => (
+            <Chip
+              key={c}
+              variant="token"
+              removeLabel={`Remove ${c}`}
+              onClick={() => setChips((all) => all.filter((x) => x !== c))}
+            >
+              {c}
+            </Chip>
+          ))}
+        </div>
+      );
+    }
+    render(<Row />);
+    const music = screen.getByRole("button", { name: "Remove Music" });
+    act(() => music.focus());
+    fireEvent.click(music, { detail: 0 });
+    expect(screen.queryByRole("button", { name: "Remove Music" })).toBeNull();
+    expect(document.activeElement).toBe(screen.getByRole("button", { name: "Remove News" }));
+  });
+
   it("the measuring mirror lays out exactly like the chip it measures", () => {
     render(
       <Chip variant="tag" size="xs">