Reservation auto-confirm setting + WhatsApp click-to-chat

New setting dd_reservation_require_confirmation (default 1, preserving current behaviour). When off, no-deposit reservations are confirmed on submission instead of arriving as pending.

Admin reservation email now renders a status-driven pill (PENDING / CONFIRMED) and drops the admin CTA entirely when the booking is already confirmed — nothing to action. A WhatsApp click-to-chat button is added in both cases, opening a conversation with the customer with the booking reference prefilled.

Numbers are stored E.164 across multiple countries; the button renders only for a leading '+' and 8-15 digits, and fails safe otherwise. href escaped with esc_attr() per the esc_url %0A stripping rule.

Deposit-required bookings are unaffected and still insert as pending.

Known gap, documented not fixed: auto-confirm leaves no activity-log entry, as DD_Activity_Module::log() requires a staff user and the customer endpoint is nopriv. A system-actor concept is needed and is out of scope here.