Add CypherGoat exchange integration

[4rkal]

Adds CypherGoat as a swap provider, following the same pattern as the Trocador integration.

• New exchange service in lib/services/exchange/cyphergoat/ with API client (/estimate, /swap, /transaction) and response models
• Wired into the exchange factory, currency loading, exchange form, provider options, and exchange icons
• Prebuild scripts generate kCypherGoatApiKey and kCypherGoatAffiliate constants

Notes:

• Not added to exchangesWithTorSupport since CypherGoat has no .onion endpoint
• assets/svg/exchange_icons/cyphergoat.svg needs to be added to the private assets pipeline (the directory is gitignored); happy to provide the SVG