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/playlistView.ts

@@ -20,7 +20,7 @@ export function applyView<T extends Grouped>(
   rows: T[],
   columns: Sortable<T>[],
   sort: SortState,
-  group: boolean
+  group: boolean,
 ): T[] {
   const sorted = sortRows(rows, columns, sort);
   if (!group) return sorted;