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/ViewSwitcher.tsx

@@ -52,7 +52,7 @@ export default function ViewSwitcher<T extends string>({
   // nothing. The menu then shows no checked item, which is honest — nothing IS selected.
   const currentIdx = Math.max(
     0,
-    options.findIndex((o) => o.id === value)
+    options.findIndex((o) => o.id === value),
   );
   // Seeding the roving index HERE — rather than in an effect keyed on `open` — matters: an effect
   // would only schedule the update, so the focus effect below would still see the previous index
@@ -111,7 +111,9 @@ export default function ViewSwitcher<T extends string>({
       >
         <CurrentIcon className="w-4 h-4 shrink-0" />
         <span className={clsx("text-sm whitespace-nowrap", labelClassName)}>{current.label}</span>
-        <ChevronDown className={`w-3 h-3 shrink-0 transition-transform ${open ? "rotate-180" : ""}`} />
+        <ChevronDown
+          className={`w-3 h-3 shrink-0 transition-transform ${open ? "rotate-180" : ""}`}
+        />
       </button>
       {open && (
         <div