fix(ui): a disabled menu row says why it is off, under its label

The reason for a disabled menu row lived in a `title` on a natively disabled
button: no pointer events, so no tooltip, and roving focus skips the row —
four translated sentences reached nobody (the Watch later rename/delete
lock, "no other list to move to", the reorder gate). An aria-disabled row was
tried and reverted in E10's review. ActionMenu items now take a `note`,
rendered as a muted line under a disabled row's label: seen, and readable in
a screen reader's browse mode. Only the icon and label dim — row-level
opacity would dim the explanation too. The reorder group states its reason
once, under its first row.

frontend/src/components/Playlists.tsx

@@ -772,17 +772,14 @@ export default function Playlists() {
                     })),
                   }
                 : // Kept visible and disabled rather than dropped: an action that vanishes teaches
-                  // nobody why. KNOWN GAP: the `title` that carries the why currently reaches nobody
-                  // either — see ActionMenu's `title` prop, where the two failed attempts at fixing
-                  // that are recorded. The row is honest about being off; it is not yet honest about
-                  // the reason.
+                  // nobody why — and its `note` says the why, under the label.
                   {
                     id: "move",
                     label: t("playlists.moveTo"),
                     icon: <FolderInput className="w-4 h-4" />,
                     separatorBefore: !recentMoveTarget,
                     disabled: true,
-                    title: t("playlists.moveNoTargets"),
+                    note: t("playlists.moveNoTargets"),
                     onSelect: () => {},
                   },
               ...(
@@ -797,11 +794,10 @@ export default function Playlists() {
                 label,
                 icon,
                 separatorBefore: i === 0,
-                // Same gate as the drag handle. NOTE the drag handle explains itself and this row
-                // does not: the handle is `aria-disabled` (focusable, tooltip-capable) while a menu
-                // row cannot be — see ActionMenu's `title` prop for why.
+                // Same gate as the drag handle. The reason goes under the FIRST of the four rows
+                // only — one explanation for the group, not the same line four times.
                 disabled: !storedOrderEditable,
-                title: storedOrderEditable ? undefined : dragHint,
+                note: i === 0 ? dragHint : undefined,
                 onSelect: () => moveInOrder(ctxRow, to),
               })),
               {

frontend/src/components/PlaylistsRail.tsx

@@ -199,7 +199,7 @@ export default function PlaylistsRail({
   // Interpolated, not spelled out: the HU copy said "Watch later", a name the Hungarian UI never
   // shows for this list ("Megnézem később") — so the message named a list the reader had never seen
   // (E10 review round 1). Passing the localized name makes the two impossible to drift.
-  const lockedTitle = ctxBuiltin
+  const lockedNote = ctxBuiltin
     ? t("playlists.builtinLocked", { name: t("playlists.watchLater") })
     : undefined;
   const railItems: ActionMenuItem[] = !ctxPl
@@ -210,7 +210,7 @@ export default function PlaylistsRail({
           label: t("playlists.rename"),
           icon: <Pencil className="w-4 h-4" />,
           disabled: ctxBuiltin,
-          title: lockedTitle,
+          note: lockedNote,
           onSelect: () => {
             setRenameValue(ctxPl.name);
             setRenamingId(ctxPl.id);
@@ -224,7 +224,7 @@ export default function PlaylistsRail({
           separatorBefore: true,
           danger: true,
           disabled: ctxBuiltin,
-          title: lockedTitle,
+          note: lockedNote,
           // The same dialog (and the same "what gets selected next" rule) as the detail header's
           // trash button — one implementation, two entry points.
           onSelect: () => void plActions.remove(ctxPl),

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

@@ -42,6 +42,22 @@ describe("ActionMenu", () => {
 
   // floating-ui never computes a position in jsdom, so the surface stays visibility:hidden; the items
   // are in the DOM but the a11y tree treats them as hidden — hence `{ hidden: true }` on role queries.
+  it("says why a disabled row is off, under its label — and only on a disabled row", () => {
+    // A `title` on a disabled row reached nobody: a disabled button fires no pointer events and
+    // roving focus skips it. The reason is visible text now, undimmed so it can be read.
+    renderMenu([
+      { id: "move", label: "Move to", onSelect: vi.fn(), disabled: true, note: "No other list" },
+      { id: "sync", label: "Sync", onSelect: vi.fn(), note: "never shown" },
+    ]);
+    fireEvent.click(screen.getByRole("button", { name: "Catalog actions" }));
+    const [move, sync] = screen.getAllByRole("menuitem", { hidden: true });
+    expect(move!.textContent).toBe("Move toNo other list");
+    expect(sync!.textContent).toBe("Sync");
+    const note = screen.getByText("No other list");
+    expect(note.closest(".opacity-50")).toBeNull();
+    expect(screen.getByText("Move to").className).toContain("opacity-50");
+  });
+
   it("opens on click and shows one menuitem per item", () => {
     renderMenu([
       { id: "sync", label: "Sync", onSelect: vi.fn() },

frontend/src/components/ui/ActionMenu.tsx

@@ -16,24 +16,37 @@ import { closeThenAct, useAnchoredPopover } from "./anchoredPopoverHooks";
 /** Shared surface paint for every menu built on this file — the ⋯ menu, its submenus, and
  *  `ContextMenu`. One constant so a context menu can never look like a different component. */
 export const MENU_SURFACE = "glass-menu min-w-52 rounded-xl p-1.5 animate-[popIn_0.16s_ease]";
+// No `disabled:opacity-50` on the row itself: a disabled row may carry a `note`, and opacity on the
+// row would dim the one line that explains it. The icon and the label dim instead (`dim`).
 const ROW =
-  "flex w-full items-center gap-2.5 rounded-lg px-2 py-1.5 text-left text-sm outline-hidden transition disabled:opacity-50";
+  "flex w-full items-center gap-2.5 rounded-lg px-2 py-1.5 text-left text-sm outline-hidden transition";
+const dim = (disabled: boolean | undefined) => (disabled ? " opacity-50" : "");
+
+/** The label, and under it — on a disabled row only — the reason it is off. */
+function RowLabel({ item }: { item: MenuItemBase }) {
+  return (
+    <span className="min-w-0 flex-1">
+      <span className={`block truncate${dim(item.disabled)}`}>{item.label}</span>
+      {item.disabled && item.note && (
+        <span className="block text-2xs leading-snug text-muted">{item.note}</span>
+      )}
+    </span>
+  );
+}
 
 interface MenuItemBase {
   id: string;
   label: string;
   icon?: ReactNode; // an element (so a caller can swap in a spinning icon while pending)
   disabled?: boolean;
-  /** Optional native tooltip (carries a longer hint the label alone can't).
-   *
-   *  KNOWN GAP, deliberately still open: on a `disabled` row this reaches nobody — a disabled button
-   *  fires no pointer events, so no tooltip, and floating-ui's roving focus skips it. E10 review
-   *  round 1 tried an `aria-disabled` "inert" row to fix that; round 2 found `useListNavigation`
-   *  skips `aria-disabled` identically (`isListIndexDisabled`), which AnchoredPopover's own comment
-   *  already said — so the swap fixed nothing and made the mouse worse (the row took the active
-   *  highlight, then no-opped on click). Reverted. A real fix has to put the reason somewhere a
-   *  disabled row is not: a visible hint line in the menu, or an `aria-describedby` note. */
+  /** Optional native tooltip (carries a longer hint the label alone can't) — for an ENABLED row. */
   title?: string;
+  /** Why a DISABLED row is off, shown as a line under its label. Not a `title`: a disabled button
+   *  fires no pointer events (no tooltip) and roving focus skips it, so a reason there reached
+   *  nobody. An `aria-disabled` row was tried and reverted (E10 review rounds 1–2: floating-ui skips
+   *  it identically, and the mouse got a highlight that then no-opped). Visible text is the one
+   *  place a disabled row's reason can be: seen, and read in a screen reader's browse mode. */
+  note?: string;
   /** Draw a divider above this item. Ignored on the first item — a menu never opens with a rule. */
   separatorBefore?: boolean;
 }
@@ -88,11 +101,13 @@ export function MenuItemList({
               className={`${ROW}${it.danger ? " text-danger" : ""}`}
             >
               {it.icon && (
-                <span className={`shrink-0 ${it.danger ? "text-danger" : "text-muted"}`}>
+                <span
+                  className={`shrink-0 ${it.danger ? "text-danger" : "text-muted"}${dim(it.disabled)}`}
+                >
                   {it.icon}
                 </span>
               )}
-              <span className="flex-1 truncate">{it.label}</span>
+              <RowLabel item={it} />
             </PopoverItem>
           )}
         </Fragment>
@@ -142,8 +157,10 @@ function SubMenu({ item, onClose }: { item: SubmenuItem; onClose: () => void })
         }}
         className={ROW}
       >
-        {item.icon && <span className="shrink-0 text-muted">{item.icon}</span>}
-        <span className="flex-1 truncate">{item.label}</span>
+        {item.icon && (
+          <span className={`shrink-0 text-muted${dim(item.disabled)}`}>{item.icon}</span>
+        )}
+        <RowLabel item={item} />
         <ChevronRight className="h-4 w-4 shrink-0 text-muted" aria-hidden="true" />
       </PopoverItem>
       <PopoverSurface