fix(a11y): address R9 code-review findings (glass shadow, focus dedup)

- #1: remove the 9 dead `shadow-*` from glass surfaces instead of letting them override the tuned glass
  box-shadow. The @layer components move made a JSX shadow-* win AND atomically drop the glass inset
  top-edge highlight; deleting the (always-dead) utilities keeps the intended glass depth incl. the
  inset sheen. Verified in-browser: a .glass box-shadow again includes its inset segment.
- #3: drop the per-element focus-visible outlines from the rail chrome buttons and SavedViews action
  buttons — the global button:focus-visible fallback already draws the identical 2px accent ring, so
  they were redundant. Verified: the pin button rings from the global rule alone.
- #5: unify the focus-within outline offset (offset-2) on the SearchBar and ShareDialog pills.
- #2 (double outline on the pill when an inner button is focused): NOT fixed — the clean fix
  (has-[input:focus] so only the input rings the pill) does not generate under Tailwind v4 (measured:
  no rule emitted). Kept the working focus-within, which is a11y-correct (every element still shows a
  focus indicator); the overlapping ring is an accepted cosmetic edge-case.
- #4 (weak ring on solid-fill selected items): deferred to R10 with the selected-paint tokenisation.

Gate green (tsc/eslint/prettier/knip/vitest 79).

frontend/src/components/AddToPlaylist.tsx

@@ -98,7 +98,7 @@ export default function AddToPlaylist({
         <ListPlus className="w-4 h-4" />
       </button>
 
-      <PopoverSurface popover={pop} className="glass-menu w-60 rounded-xl p-2 shadow-2xl">
+      <PopoverSurface popover={pop} className="glass-menu w-60 rounded-xl p-2">
         {/* A React portal still propagates events up the REACT tree, so a click inside this panel
             would otherwise reach the feed card that hosts the trigger and start playback. */}
         <div onClick={(e) => e.stopPropagation()}>