Fix exchange rates: migrate from frankfurter.app to frankfurter.dev

[stefantaubert]

Summary

Fixes #514

The Frankfurter API has moved from api.frankfurter.app to api.frankfurter.dev.
The old domain returns a 301 redirect without CORS headers, which causes the browser
to block exchange rate requests.

Problem

• https://api.frankfurter.app/2026-04-05?base=CNY → 301 redirect to api.frankfurter.dev
• The 301 response is missing the Access-Control-Allow-Origin header
• Browser blocks the request due to CORS policy

Fix

Replace api.frankfurter.app with api.frankfurter.dev/v1 in the exchange rate client.

Note: frankfurter.dev also offers a v2 API, but it has a different response schema.
This PR intentionally uses /v1/ to maintain compatibility without requiring
additional changes to the response parsing logic.

Test plan

• Open a group
• Verify exchange rates load without CORS errors