Auto-apply Promo Codes for Direct Bookings

Allow hosts to automatically apply promo codes to the Direct booking widget so guests don't have to enter them manually. This should include:

• The ability to auto-apply a specific code when the widget loads.
• Support for URL parameters (e.g., ?promo_code=CODE) to apply discounts via shareable links for ads or marketing.
• An option to pre-fill the discount code field.

Business impact: Hosts running paid ads (Google/Meta) to their Direct site lose conversions because guests must manually enter the code. Many either pay full price and feel misled, or abandon the booking — which undercuts the whole 'book direct and save' value proposition and wastes ad spend.