feat(uat-batch-20): form-error UX primitive — scroll-to-first-error hook + summary banner

Two new building blocks for the platform-wide form-error UX rework.
Expense form adopts both as the validation that the pattern works
before the broader sweep across the ~29 useForm callers.

- `useFormScrollToError(handleSubmit, errors)` — wraps RHF's
  handleSubmit. On validation failure it locates the first errored
  field via `[name="..."]` (or id fallback), walks ancestors to find
  the nearest scrolling container (key for forms inside Sheet /
  Dialog bodies that own their own overflow-y), and
  scrollTo({ behavior: 'smooth' }) + focus({ preventScroll }) on it.
  Type-loose handleSubmit signature so 2-arg and 3-arg useForm()
  callers (input vs transformed types) both work.
- `<FormErrorSummary errors={errors} labels={…}>` — top-of-form alert
  banner listing each failed field as a clickable anchor. Renders
  only when ≥2 errors (single-error case is handled by the hook
  alone). role="alert" aria-live="polite" for SR users.
- expense-form-dialog adopts both: `onSubmitWithScroll(onSubmit)`
  replaces the bare `handleSubmit(onSubmit)`, plus a labelled
  `<FormErrorSummary>` at the top of the form. Closes the loop on
  the silent-no-op zod-refine bug fixed in PR1 (the underlying
  setValue() fix already routes errors through formState; this
  surfaces them visibly).

tsc clean. 1419/1419 vitest pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/hooks/use-form-scroll-to-error.ts

@@ -0,0 +1,90 @@
+'use client';
+
+import { useCallback } from 'react';
+import type { FieldErrors, FieldValues } from 'react-hook-form';
+
+// react-hook-form's handleSubmit is generic across the input vs.
+// transformed types. We don't need the strictness here — the wrapper
+// just passes its handler through to whatever handleSubmit the caller
+// gave us. Use a loose type so 2-arg and 3-arg useForm() both work.
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+type AnyHandleSubmit = (
+  onValid: any,
+  onInvalid?: any,
+) => (e?: React.BaseSyntheticEvent) => Promise<void>;
+
+/**
+ * Find the nearest scrolling ancestor of a node — accounts for the
+ * common case of forms rendered inside a Sheet / Dialog body that owns
+ * its own overflow-y. Falls back to `window` when no ancestor scrolls.
+ */
+function findScrollContainer(el: HTMLElement | null): HTMLElement | null {
+  let cur: HTMLElement | null = el?.parentElement ?? null;
+  while (cur) {
+    const style = window.getComputedStyle(cur);
+    const overflowY = style.overflowY;
+    if ((overflowY === 'auto' || overflowY === 'scroll') && cur.scrollHeight > cur.clientHeight) {
+      return cur;
+    }
+    cur = cur.parentElement;
+  }
+  return null;
+}
+
+/**
+ * Wrap react-hook-form's `handleSubmit` so validation failures scroll
+ * the first errored field into view and focus it. Critical on tall
+ * drawers / dialogs where the failing field is below the fold —
+ * without this the user is dropped at the top of the form with no
+ * indication of what failed.
+ *
+ * Usage:
+ * ```
+ * const { handleSubmit, formState: { errors }, ... } = useForm(...);
+ * const onSubmit = useFormScrollToError(handleSubmit, errors);
+ * <form onSubmit={onSubmit(myHandler)}>...</form>
+ * ```
+ *
+ * `errors` is taken from `formState` so the hook reads the FIRST key
+ * (insertion order matches field render order in practice).
+ */
+export function useFormScrollToError<TFieldValues extends FieldValues>(
+  handleSubmit: AnyHandleSubmit,
+  errors: FieldErrors<TFieldValues>,
+) {
+  return useCallback(
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    (onValid: (data: any) => void | Promise<void>) => {
+      return handleSubmit(onValid, () => {
+        // react-hook-form calls this on validation failure. We already
+        // have `errors` from formState — read the first key and scroll
+        // its DOM node into view.
+        const firstName = Object.keys(errors)[0];
+        if (!firstName) return;
+        // Find by `name` first (most input components forward `name`
+        // from `register`), then by `id` (fallback for custom Inputs).
+        const node =
+          (document.querySelector(`[name="${firstName}"]`) as HTMLElement | null) ??
+          (document.getElementById(firstName) as HTMLElement | null);
+        if (!node) return;
+        const container = findScrollContainer(node);
+        if (container) {
+          // Manually compute position so we scroll inside the
+          // container, not the page.
+          const cRect = container.getBoundingClientRect();
+          const nRect = node.getBoundingClientRect();
+          const offset = nRect.top - cRect.top + container.scrollTop - cRect.height / 2;
+          container.scrollTo({ top: offset, behavior: 'smooth' });
+        } else {
+          node.scrollIntoView({ block: 'center', behavior: 'smooth' });
+        }
+        if (typeof node.focus === 'function') {
+          // Defer focus until after the smooth scroll has started so
+          // the focus ring is visible.
+          window.setTimeout(() => node.focus({ preventScroll: true }), 50);
+        }
+      });
+    },
+    [handleSubmit, errors],
+  );
+}