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/ColumnSortControl.tsx

@@ -56,7 +56,11 @@ export default function ColumnSortControl<T>({
           aria-label={active.dir === "asc" ? ascLabel : descLabel}
           className="shrink-0 p-1.5 rounded-lg border border-border bg-card text-fg hover:border-accent hover:text-accent active:translate-y-px transition"
         >
-          {active.dir === "asc" ? <ArrowUp className="w-4 h-4" /> : <ArrowDown className="w-4 h-4" />}
+          {active.dir === "asc" ? (
+            <ArrowUp className="w-4 h-4" />
+          ) : (
+            <ArrowDown className="w-4 h-4" />
+          )}
         </button>
       )}
     </div>