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/Toaster.tsx

@@ -1,24 +1,9 @@
 import { useSyncExternalStore } from "react";
 import { useTranslation } from "react-i18next";
-import {
-  AlertCircle,
-  AlertTriangle,
-  CheckCircle2,
-  Info,
-  X,
-  XCircle,
-} from "lucide-react";
-import {
-  dismiss,
-  getActiveToasts,
-  subscribe,
-  type NotifLevel,
-} from "../lib/notifications";
+import { AlertCircle, AlertTriangle, CheckCircle2, Info, X, XCircle } from "lucide-react";
+import { dismiss, getActiveToasts, subscribe, type NotifLevel } from "../lib/notifications";
 
-export const LEVEL_STYLE: Record<
-  NotifLevel,
-  { icon: typeof Info; color: string; bar: string }
-> = {
+export const LEVEL_STYLE: Record<NotifLevel, { icon: typeof Info; color: string; bar: string }> = {
   info: { icon: Info, color: "text-accent", bar: "bg-accent" },
   success: { icon: CheckCircle2, color: "text-emerald-400", bar: "bg-emerald-400" },
   warning: { icon: AlertTriangle, color: "text-amber-400", bar: "bg-amber-400" },