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/feedSort.ts

@@ -34,10 +34,7 @@ const SORT_MAP: Record<Exclude<SortKey, "shuffle" | "relevance">, { asc: string;
   priority: { desc: "priority", asc: "priority_asc" },
 };
 
-export const SORT_DEFAULT_DIR: Record<
-  Exclude<SortKey, "shuffle" | "relevance">,
-  "asc" | "desc"
-> = {
+export const SORT_DEFAULT_DIR: Record<Exclude<SortKey, "shuffle" | "relevance">, "asc" | "desc"> = {
   date: "desc",
   popular: "desc",
   duration: "desc",