IAV-1231 : add credit conversion system with EUR→MAD auto-calculation

• Add ConversionParameters component (realExchangeRate, majoredExchangeRate)
• Refactor ConversionRates to show charge, gain%, marge, valeur de crédit and auto-computed taux de conversion
• Auto-calculate MAD price, charge and credit fields from EUR × majoredExchangeRate
• Lock MAD fields (price, charge, gain%) when majoredExchangeRate is set
• Add isLoadingEdit guard to prevent auto-calc overwriting values in edit mode
• Pass full product object in PlansTab onEdit instead of partial reconstruction
• Add credit/conversion fields to types, store and API payloads
• Restructure discount section into EUR/MAD grid layout
• Fix subscription card name and discount badge positioning