fix: Spanish frontend, wider columns, currency symbols, PDF download

PDF fix:
- Replace session-cookie auth with HTTP Basic Auth (username:api_key)
  which is natively supported by Odoo 17+ report endpoints
- Validate response is actually a PDF (%PDF header check) before serving
- Return a descriptive Spanish error if HTTP code != 200 or body is not PDF

Frontend → Spanish:
- All invoice template text in Spanish (Nº Factura, Vencimiento, etc.)
- All calendar/booking template text in Spanish
- Payment state labels: Pendiente / Parcial / En cobro / Pagado / Anulado
- "Vencida" badge for overdue invoices
- Error/notice messages in Spanish across both pages

Currency symbols:
- Add currency_symbol() helper mapping ISO codes to symbols
- EUR → €, USD → $, GBP → £, etc. (25 currencies mapped)

Column widths:
- Switch invoice table to table-layout:fixed with explicit column widths
- col-number: 180px nowrap so reference never wraps across lines
- Date/due/amount/status columns all fixed-width and nowrap
- Add .woodoo-nowrap utility class

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

includes/class-woodoo-calendar.php

@@ -41,7 +41,7 @@ class WooDoo_Calendar {
         $api = woodoo_api();
         if ( ! $api ) {
             echo '<p class="woodoo-notice woodoo-error">' .
-                 esc_html__( 'Booking is not available right now. Please contact support.', 'woodoo' ) .
+                 'Las reservas no están disponibles en este momento. Por favor, contacta con soporte.' .
                  '</p>';
             return;
         }