Add currency signs to /v1/currencies endpoint

[Copilot]

• Add CurrencyInfo struct with Name and Sign fields to handlers/service.go
• Change CurrencyNames map from map[string]string to map[string]CurrencyInfo with currency signs
• Add Sign field to NamedSymbol struct
• Update GetAllNamedSymbols() to include sign in response (append directly in if-ok clause)
• Update unit tests in handlers/named_symbols_test.go to verify sign field
• Update integration test TestGetCurrenciesEndpoint to verify sign field
• Update README.md with updated /v1/currencies example response showing sign field

Original prompt

Expand the currencies endpoint with currency signs, for example Euro is €.
Make sure this covered by the existing integration to clearly demonstrate how this feature works.
You probably need to change the structure of the symbols map to make this work efficiently.

Once done update the README with up to date docs.

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[kamaal111]

@copilot don't juggle with empty values just append in the if ok clause

[Copilot]

Done in c3cfc8e — the loop now appends directly inside the if ok clause, removing the intermediate variables.