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/clients/bulk-hard-delete-dialog.tsx

@@ -182,8 +182,12 @@ function BulkHardDeleteDialogBody({ onOpenChange, clientIds, onDeleted }: Props)
             <table className="w-full text-xs">
               <thead className="bg-muted/50 sticky top-0">
                 <tr>
-                  <th className="text-left px-2 py-1.5 font-medium">Client ID</th>
-                  <th className="text-left px-2 py-1.5 font-medium">Reason</th>
+                  <th scope="col" className="text-left px-2 py-1.5 font-medium">
+                    Client ID
+                  </th>
+                  <th scope="col" className="text-left px-2 py-1.5 font-medium">
+                    Reason
+                  </th>
                 </tr>
               </thead>
               <tbody>