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/index.css

@@ -280,10 +280,11 @@ body {
 }
 
 /* ===== Liquid-glass surface system (theme-aware, GPU-light) ===== */
-/* In @layer components so a JSX shadow-* utility (which lives in @layer utilities, always above
-   components) can override a glass surface's default box-shadow again. Before v4/R9 these rules were
-   unlayered and always won, so ~9 `shadow-*` on glass surfaces were silent no-ops. The perf-mode and
-   :hover overrides below stay UNLAYERED on purpose (they must beat the utility). */
+/* In @layer components so the recipe classes below (.selectable) compose with utilities instead of
+   silently beating them the way these rules did while unlayered. (The ~9 dead `shadow-*` that used to
+   sit on glass surfaces were REMOVED in R9, not activated: the tuned glass box-shadow — including its
+   inset top-edge highlight — is the intended depth, and a shadow-* utility would atomically replace it
+   and drop that highlight.) The perf-mode and :hover overrides below stay UNLAYERED on purpose. */
 @layer components {
   .glass {
     /* Frosted overlay surface: translucent enough that the blurred backdrop shows