fix(design): claim audit part 1 — a closing key no longer re-presses its trigger, the Switch box and name, named z-levels (E28)

- In-place editors (Playlists heading + rail rename, Plex collection + playlist rename, PriorityStepper,
  Scheduler interval): a real Enter closed the editor, returned the focus to the trigger, and the same
  keystroke's activation pressed that trigger again - re-opening the editor, or in the rail selecting
  the renamed playlist. useReturnFocus.arm(e) now cancels the key's default (type-enforced); the rail and
  Scheduler editors do the same. The earlier tests dispatched a synthetic keydown, which has no default.
- Plex playlist rename: an unchanged or empty title closes with no write, as the collection rename does.
- Switch: a 36x24 box around the track (the 24 floor), 44 on a coarse pointer; `label` is required, and
  the Plex two-way-sync switch had no name.
- z-index: numeric z-10/z-20 and a literal 40 moved onto the named layer scale; a lint rule + contract.
- The /watch file card's Download is the filled primary: the page's one action.
- Stale comments: the 1024x768 floor, the retired glass tuner.

frontend/src/components/ui/Carousel.tsx

@@ -99,7 +99,7 @@ export function Carousel({
   // removes it on touch, where the strip scrolls by swipe — so an invisible edge button never eats a tap.
   // Round, dark translucent: they sit over the pictures (A2).
   const arrowBtn =
-    "reveal-hover absolute top-1/2 -translate-y-1/2 z-10 backdrop-blur-xs border border-white/15 shadow-lg " +
+    "reveal-hover absolute top-1/2 -translate-y-1/2 z-base backdrop-blur-xs border border-white/15 shadow-lg " +
     "opacity-0 group-hover/car:opacity-100 focus-visible:opacity-100";
 
   return (