feat(design): reorder without a drag everywhere a grip drags (WCAG 2.5.7, E28 S3e4a)

One rule, two forms: useOrderItems (Top / Up / Down / Bottom rows of a row
menu) and OrderButtons (four buttons with the place, or a compact Up / Down
pair). A move that would go nowhere is shown off.

- YouTube playlist rows: the four moves now come from useOrderItems.
- Plex playlist rows: a block among the blocks, an episode within its show,
  off while a write is in flight; drag and menu share moveBlock/moveEpisode.
- Saved views editor: the selected view's four buttons (owner: A), and the
  grip's keyboard pick-up (its instructions promised one it did not have).
- Side panel groups in edit mode: an Up / Down pair (owner: A); a group the
  stored order does not hold yet moves too; the name wraps there.
- The grip's touch-action: none is now test-held.

frontend/src/lib/moveInList.test.ts

@@ -0,0 +1,24 @@
+import { describe, expect, it } from "vitest";
+import { moveTarget } from "./moveInList";
+
+describe("moveTarget", () => {
+  it("sends an item to the top, one up, one down, or to the bottom", () => {
+    expect(moveTarget(2, 5, "top")).toBe(0);
+    expect(moveTarget(2, 5, "up")).toBe(1);
+    expect(moveTarget(2, 5, "down")).toBe(3);
+    expect(moveTarget(2, 5, "bottom")).toBe(4);
+  });
+
+  it("is null for a move that goes nowhere: the first up, the last down", () => {
+    expect(moveTarget(0, 5, "top")).toBeNull();
+    expect(moveTarget(0, 5, "up")).toBeNull();
+    expect(moveTarget(4, 5, "down")).toBeNull();
+    expect(moveTarget(4, 5, "bottom")).toBeNull();
+    expect(moveTarget(0, 1, "down")).toBeNull();
+  });
+
+  it("is null for an index outside the list", () => {
+    expect(moveTarget(-1, 5, "top")).toBeNull();
+    expect(moveTarget(5, 5, "up")).toBeNull();
+  });
+});

frontend/src/lib/moveInList.ts

@@ -0,0 +1,14 @@
+// The non-drag way to reorder a list (WCAG 2.5.7, docs/DESIGN.md "Reordering"): anything a grip can
+// drag also moves by Top / Up / Down / Bottom — a menu's rows or a row of buttons, one rule for both.
+
+export type MoveTo = "top" | "up" | "down" | "bottom";
+
+export const MOVES: readonly MoveTo[] = ["top", "up", "down", "bottom"];
+
+/** Where `to` sends the item at `from` in a list of `count`, or null when it would not move (the
+ *  first item going up, the last going down, a stale index) — the caller shows that move as off. */
+export function moveTarget(from: number, count: number, to: MoveTo): number | null {
+  if (from < 0 || from >= count) return null;
+  const at = to === "top" ? 0 : to === "bottom" ? count - 1 : to === "up" ? from - 1 : from + 1;
+  return at === from || at < 0 || at >= count ? null : at;
+}