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/ui/calendar.tsx

@@ -32,7 +32,10 @@ function Calendar({
       )}
       captionLayout={captionLayout}
       formatters={{
-        formatMonthDropdown: (date) => date.toLocaleString('default', { month: 'short' }),
+        // `undefined` honours the browser locale. The previous
+        // `'default'` literal is treated as undefined by every modern
+        // engine but reads as if a specific locale were intended.
+        formatMonthDropdown: (date) => date.toLocaleString(undefined, { month: 'short' }),
         ...formatters,
       }}
       classNames={{