feat(uat-batch-19): a11y th scopes + legend styling + i18n locale fixes

- Raw `<th>` cells gain `scope="col"` so SR users get proper column
  association: berth-interests-tab, bulk-add-berths-wizard,
  clients/bulk-hard-delete-dialog. shadcn `<TableHead>` migration
  would be cleaner but the scope attribute is the minimum-effort fix
  the queue's a11y entry asks for.
- supplemental-info form `<legend>` elements styled with
  `mb-2 px-1 font-semibold` so they read as section headings rather
  than blending into the surrounding fieldset border (default browser
  legend rendering is barely visible).
- payments-section: invalid `'en-EU'` BCP-47 locale → `undefined` to
  honour browser locale.
- ui/calendar: literal `'default'` → `undefined` on the month
  dropdown formatter, same reason.

tsc clean. 1419/1419 vitest pass.

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

src/components/interests/payments-section.tsx

@@ -64,7 +64,10 @@ function formatMoney(amount: string, currency: string): string {
   const n = Number(amount);
   if (!Number.isFinite(n)) return `${amount} ${currency}`;
   try {
-    return new Intl.NumberFormat('en-EU', { style: 'currency', currency }).format(n);
+    // `undefined` locale honours the user's browser locale. The
+    // previous `'en-EU'` literal is not a valid BCP-47 tag — every
+    // implementation falls back to the default anyway.
+    return new Intl.NumberFormat(undefined, { style: 'currency', currency }).format(n);
   } catch {
     return `${n.toFixed(2)} ${currency}`;
   }