Support currency configuration (e.g. GBP/£ instead of USD/$)

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Feature Request

It would be great to support configurable currency in the cost module. Currently the cost is displayed in USD with a $ symbol, but for users outside the US (e.g. UK), it would be useful to:

1. Configure the currency symbol (e.g. £ instead of $)
2. Optionally apply a conversion rate

Proposed config

[cship.cost]
currency_symbol = "£"
conversion_rate = 0.79  # optional, USD to GBP

This would allow international users to see costs in their local currency.

Thanks for the great tool!