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/app/public/supplemental-info/[token]/page.tsx

@@ -195,7 +195,7 @@ export default function SupplementalInfoPage({ params }: PageProps) {
         </div>
 
         <fieldset className="space-y-4">
-          <legend className="text-sm font-semibold">Your details</legend>
+          <legend className="mb-2 px-1 text-sm font-semibold text-foreground">Your details</legend>
           <div className="space-y-1.5">
             <Label htmlFor="fullName">Full name</Label>
             <Input
@@ -244,7 +244,9 @@ export default function SupplementalInfoPage({ params }: PageProps) {
         </fieldset>
 
         <fieldset className="space-y-4">
-          <legend className="text-sm font-semibold">Your yacht (optional)</legend>
+          <legend className="mb-2 px-1 text-sm font-semibold text-foreground">
+            Your yacht (optional)
+          </legend>
           <div className="space-y-1.5">
             <Label htmlFor="yachtName">Yacht name</Label>
             <Input