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/PlaylistsProvider.tsx

@@ -1,11 +1,4 @@
-import {
-  createContext,
-  useCallback,
-  useContext,
-  useMemo,
-  useState,
-  type ReactNode,
-} from "react";
+import { createContext, useCallback, useContext, useMemo, useState, type ReactNode } from "react";
 import { getAccountRaw, LS, setAccountRaw } from "../lib/storage";
 
 // Owns the Playlists module's shared local state — the rail's name filter and the selected playlist