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/WatchPage.tsx

@@ -305,7 +305,8 @@ export default function WatchPage() {
           /* A generic uploaded file has no player — just an info card (name/type/size) and, if the
              link allows it, a Download. The backend serves the file force-download regardless, so
              gating the button on allow_download only mirrors the owner's ACL, matching the media
-             branch's download affordance below. */
+             branch's download affordance below. It is the page's one action, so it is the filled
+             primary (E28), as the unlock card's Watch is. */
           <div className="flex-1 grid place-items-center">
             <div className={`${SURFACE.panel} w-full max-w-md p-6 text-center`}>
               <div className="mx-auto w-16 h-16 rounded-xl bg-surface grid place-items-center mb-4">
@@ -320,7 +321,12 @@ export default function WatchPage() {
                   .join(" · ")}
               </div>
               {meta.allow_download && (
-                <LinkButton href={meta.file_url} icon={<Download />} className="mt-5">
+                <LinkButton
+                  href={meta.file_url}
+                  variant="primary"
+                  icon={<Download />}
+                  className="mt-5"
+                >
                   {T.download}
                 </LinkButton>
               )}