Pre-order - Display availability as relative timeframe + add weeks/months unit options

Two related improvements to the "Available on a Timeframe" availability setting: 1. Display as relative timeframe: Allow the availability text to show a human-readable relative format (e.g., "Available in 6 weeks") instead of converting the input to a calculated exact date (e.g., "2026-05-07"). 2. Add weeks/months as unit options: Currently only "Day(s) after purchase" is available. Add "Week(s) after purchase" and "Month(s) after purchase" as selectable units. Current behavior: Merchant enters 42 days and the widget calculates and displays an exact date. Requested behavior: Merchant selects 6 weeks and the widget displays "Available in 6 weeks" on the product page. Use case: Merchant wants to communicate estimated delivery in a more customer-friendly way without committing to a specific date that may shift. Showing "Available in 6 weeks" is more natural and flexible than an exact date.