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/storage.ts

@@ -80,7 +80,10 @@ export function activeAccountId(): number | null {
  * accounts, two tabs would fight over it). Defaults to the current tab's active account; returns
  * null when no account is known yet, so callers fall back to defaults / skip persistence instead
  * of touching another account's data. */
-export function accountKey(base: string, id: number | null | undefined = activeAccountId()): string | null {
+export function accountKey(
+  base: string,
+  id: number | null | undefined = activeAccountId(),
+): string | null {
   return id != null ? `${base}.${id}` : null;
 }