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/index.css

@@ -1035,11 +1035,10 @@ html[data-theme="light"] {
 .player-stage--max {
   position: fixed;
   inset: 0;
-  /* rail-level (40): this stage only has to beat the modal chrome it is nested inside (whose own
-     children top out at z-menu/20). The modal root is a z-overlay stacking context, so nothing
-     here escapes it — 9999 is the glass tuner's reserved level and claimed an authority this
-     layer neither has nor needs. */
-  z-index: 40;
+  /* rail level: this stage only has to beat the modal chrome it is nested inside (whose own
+     children top out at the menu level). The modal root is a z-overlay stacking context, so nothing
+     here escapes it — a higher level would claim an authority this layer neither has nor needs. */
+  z-index: var(--z-index-rail);
 }
 
 /* Thin, theme-aware scrollbars */