style(frontend): Prettier config + one-time repo-wide format pass

Prettier 3 (pinned), printWidth 100, double quotes — chosen to match the existing
hand-formatting and minimise reflow. This commit is ONLY the mechanical format pass
(`prettier --write`) plus the config/ignore/scripts, isolated so it never pollutes a
feature diff. Verified behaviour-neutral: typecheck, eslint (0 errors), and vitest
all green before and after. See .git-blame-ignore-revs — `git blame` skips this sha.

frontend/src/components/BackToTop.tsx

@@ -40,7 +40,10 @@ export default function BackToTop({ dep, threshold = 600 }: { dep?: unknown; thr
   }, [dep, threshold]);
 
   const toTop = () =>
-    (scrollerRef.current ?? document.querySelector("main"))?.scrollTo({ top: 0, behavior: "smooth" });
+    (scrollerRef.current ?? document.querySelector("main"))?.scrollTo({
+      top: 0,
+      behavior: "smooth",
+    });
 
   // No aria-label/title attributes: ad-block "annoyance" cosmetic filters (e.g. Brave Shields)
   // commonly hide floating corner buttons via attribute selectors like [aria-label="Back to top"].
@@ -55,6 +58,6 @@ export default function BackToTop({ dep, threshold = 600 }: { dep?: unknown; thr
       <ArrowUp className="w-7 h-7" aria-hidden="true" />
       <span className="sr-only">{t("common.backToTop")}</span>
     </button>,
-    document.body
+    document.body,
   );
 }