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/lib/useScrollFade.ts

@@ -10,7 +10,10 @@ import { useEffect, useState, type CSSProperties } from "react";
 // `fadeTop` gates the top edge only. Pass false when the scroller's top is a sticky element (a
 // table's sticky header): fading there would eat into the header instead of hinting at content, and
 // the header would visibly change as the fade turns on with the first scroll.
-export function useScrollFade(fadePx = 20, fadeTop = true): {
+export function useScrollFade(
+  fadePx = 20,
+  fadeTop = true,
+): {
   ref: (node: HTMLElement | null) => void;
   style: CSSProperties;
 } {
@@ -43,8 +46,10 @@ export function useScrollFade(fadePx = 20, fadeTop = true): {
     // every child of a long list on each append.
     const mo = new MutationObserver((records) => {
       for (const rec of records) {
-        for (const node of Array.from(rec.addedNodes)) if (node instanceof Element) ro.observe(node);
-        for (const node of Array.from(rec.removedNodes)) if (node instanceof Element) ro.unobserve(node);
+        for (const node of Array.from(rec.addedNodes))
+          if (node instanceof Element) ro.observe(node);
+        for (const node of Array.from(rec.removedNodes))
+          if (node instanceof Element) ro.unobserve(node);
       }
       update();
     });