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/ConfirmProvider.tsx

@@ -46,13 +46,11 @@ export function useConfirm(): ConfirmFn {
         title: opts.title,
         message: opts.message,
         cancelLabel: opts.cancelLabel,
-        choices: [
-          { id: "confirm", label: opts.confirmLabel ?? "", danger: opts.danger },
-        ],
+        choices: [{ id: "confirm", label: opts.confirmLabel ?? "", danger: opts.danger }],
       });
       return picked === "confirm";
     },
-    [ask]
+    [ask],
   );
 }
 
@@ -71,7 +69,7 @@ export function ConfirmProvider({ children }: { children: ReactNode }) {
 
   const ask = useCallback<ChoiceFn>(
     (opts) => new Promise<string | null>((resolve) => setState({ opts, resolve })),
-    []
+    [],
   );
 
   function close(id: string | null) {
@@ -100,11 +98,7 @@ export function ConfirmProvider({ children }: { children: ReactNode }) {
               side they wrap raggedly and a wrapped button reads as a second, lesser group — and
               these are peers. Ordered least to most destructive, top to bottom. */}
           <div
-            className={
-              multi
-                ? "flex flex-col gap-2 mt-5"
-                : "flex justify-end gap-2 mt-5 flex-wrap"
-            }
+            className={multi ? "flex flex-col gap-2 mt-5" : "flex justify-end gap-2 mt-5 flex-wrap"}
           >
             <button
               autoFocus={anyDanger}