fix(menus): one tick for every choice; a listbox holds its options only

- The saved views, the view switcher, the year filter and the table's single-choice filter mark the
  chosen row with MenuCheck, as DESIGN asks, not a trailing tick, a dot or a filled row; a contract
  test holds every checkable menu row to it. keepSelectedPaint had no caller left and is gone.
- A combobox's panel carries its listbox role on an inner PopoverList (listInside), so the failed
  load, Loading, No matches and the 'n more' line sit beside the options, not inside the listbox. The
  Plex search box's Loading line likewise.
- The Plex sidebar's playlist picker offers Create only once the lists have loaded: over a failed load
  it pushed the error and its Retry out at the first keystroke.

frontend/src/components/ViewSwitcher.tsx

@@ -1,8 +1,9 @@
 import { useState } from "react";
-import { Check, type LucideIcon } from "lucide-react";
+import { type LucideIcon } from "lucide-react";
 import clsx from "clsx";
 import { PopoverItem, PopoverSurface } from "./ui/AnchoredPopover";
 import { useAnchoredPopover } from "./ui/anchoredPopoverHooks";
+import { MenuCheck } from "./ui/MenuCheck";
 import { SelectTrigger } from "./ui/SelectTrigger";
 import { SURFACE } from "./ui/surfaces";
 
@@ -99,9 +100,9 @@ export default function ViewSwitcher<T extends string>({
                   on ? "text-accent" : "text-fg"
                 }`}
               >
+                <MenuCheck kind="one" on={on} />
                 <Icon className="w-4 h-4 shrink-0" />
                 <span className="flex-1 truncate">{o.label}</span>
-                {on && <Check className="w-3.5 h-3.5 shrink-0" />}
               </PopoverItem>
             );
           })}