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/Banner.tsx

@@ -5,8 +5,16 @@ import { X } from "lucide-react";
 type BannerTone = "accent" | "warning";
 
 const TONES: Record<BannerTone, { bar: string; icon: string; action: string }> = {
-  accent: { bar: "bg-accent/15 border-accent/30", icon: "text-accent", action: "bg-accent text-accent-fg" },
-  warning: { bar: "bg-amber-500/15 border-amber-500/30", icon: "text-amber-500", action: "bg-amber-500 text-black" },
+  accent: {
+    bar: "bg-accent/15 border-accent/30",
+    icon: "text-accent",
+    action: "bg-accent text-accent-fg",
+  },
+  warning: {
+    bar: "bg-amber-500/15 border-amber-500/30",
+    icon: "text-amber-500",
+    action: "bg-amber-500 text-black",
+  },
 };
 
 // Shared top-of-app notification bar. VersionBanner and DemoBanner are thin wrappers over this;