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/NavSidebar.tsx

@@ -241,7 +241,7 @@ export default function NavSidebar({
           {!slim && (
             <button
               onClick={() => setPage("feed")}
-              className="text-lg font-bold tracking-tight select-none cursor-pointer rounded-md -mx-1 px-1 hover:bg-card hover:opacity-90 transition outline-hidden focus-visible:outline-2 focus-visible:outline-accent/60"
+              className="text-lg font-bold tracking-tight select-none cursor-pointer rounded-md -mx-1 px-1 hover:bg-card hover:opacity-90 transition"
               title={t("header.feed")}
             >
               Sift<span className="text-accent">lode</span>
@@ -251,7 +251,7 @@ export default function NavSidebar({
             onClick={onToggleCollapse}
             title={pinned ? t("nav.unpin") : t("nav.pin")}
             aria-label={pinned ? t("nav.unpin") : t("nav.pin")}
-            className="p-1 rounded-md text-muted hover:text-fg hover:bg-card transition outline-hidden focus-visible:outline-2 focus-visible:outline-accent/60"
+            className="p-1 rounded-md text-muted hover:text-fg hover:bg-card transition"
           >
             {pinned ? <PinOff className="w-4 h-4" /> : <Pin className="w-4 h-4" />}
           </button>
@@ -290,7 +290,7 @@ export default function NavSidebar({
               onClick={onOpenAbout}
               title={t("header.account.about")}
               aria-label={t("header.account.about")}
-              className="p-2 rounded-lg text-muted hover:text-fg hover:bg-card transition outline-hidden focus-visible:outline-2 focus-visible:outline-accent/60"
+              className="p-2 rounded-lg text-muted hover:text-fg hover:bg-card transition"
             >
               <Info className="w-5 h-5" />
             </button>
@@ -298,7 +298,7 @@ export default function NavSidebar({
               onClick={logout}
               title={t("header.account.signOut")}
               aria-label={t("header.account.signOut")}
-              className="p-2 rounded-lg text-muted hover:text-fg hover:bg-card transition outline-hidden focus-visible:outline-2 focus-visible:outline-accent/60"
+              className="p-2 rounded-lg text-muted hover:text-fg hover:bg-card transition"
             >
               <LogOut className="w-5 h-5" />
             </button>