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/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),