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/admin/bulk-add-berths-wizard.tsx

@@ -227,14 +227,28 @@ export function BulkAddBerthsWizard() {
           <table className="w-full text-xs">
             <thead>
               <tr className="border-b text-left text-muted-foreground">
-                <th className="py-2 pr-2">Mooring</th>
-                <th className="py-2 pr-2">Length (ft)</th>
-                <th className="py-2 pr-2">Width (ft)</th>
-                <th className="py-2 pr-2">Draft (ft)</th>
-                <th className="py-2 pr-2">Side pontoon</th>
-                <th className="py-2 pr-2">Price</th>
-                <th className="py-2 pr-2">Currency</th>
-                <th className="py-2 pr-2" />
+                <th scope="col" className="py-2 pr-2">
+                  Mooring
+                </th>
+                <th scope="col" className="py-2 pr-2">
+                  Length (ft)
+                </th>
+                <th scope="col" className="py-2 pr-2">
+                  Width (ft)
+                </th>
+                <th scope="col" className="py-2 pr-2">
+                  Draft (ft)
+                </th>
+                <th scope="col" className="py-2 pr-2">
+                  Side pontoon
+                </th>
+                <th scope="col" className="py-2 pr-2">
+                  Price
+                </th>
+                <th scope="col" className="py-2 pr-2">
+                  Currency
+                </th>
+                <th scope="col" className="py-2 pr-2" />
               </tr>
               <tr className="border-b bg-muted/30">
                 <td className="py-1 pr-2 text-[10px] text-muted-foreground">apply to all →</td>