FlipperHelper: Resell Tracker

Added - Return deadlines. Mark an item as returnable with a return date; the item list shows a countdown badge next to the price, turning red at ≤3 days and reading "Too late" once the window has passed. - "Days to Sell" basis. Choose whether time-to-sell is measured from the purchase date or from the date you listed it (falling back to purchase date for offline/car-boot sales that were never listed on the platform they sold on). - Customizable Add Item fields. Show only the fields you use — Title, Market, Seller, Purchase Date, Return Date — via a "Customize fields" sheet opened right from the New Item screen, each with a one-line description. (Price and payment always show.) Fixed - Saving a haul with no items now shows a short explanation that a haul needs items to track a trip's performance, instead of a silently greyed-out Save button. - Long prices no longer wrap onto a second line when the return badge is shown.