feat: replace unofficial currency API with official Central Bank APIs

[konard]

Summary

Replace the unofficial fawazahmed0/currency-api with official Central Bank APIs:

• European Central Bank (ECB) via frankfurter.dev for most currencies
• Central Bank of Russia (CBR) via cbr.ru for RUB rates

This ensures all exchange rate data comes from trusted, official sources with known data provenance.

Changes

API Migration

• Updated currency_api.rs to use the Frankfurter API (which sources data from ECB)
• Changed API response parsing to match Frankfurter's JSON format
• Updated all source attribution strings from fawazahmed0/currency-api to frankfurter.dev (ECB) or cbr.ru (Central Bank of Russia)

Automated Rate Updates

• Added new GitHub Actions workflow (.github/workflows/update-currency-rates.yml)
• Runs weekly on Sunday at 00:00 UTC
• Can be triggered manually via workflow_dispatch
• Downloads fresh rates from both ECB and CBR
• Commits changes automatically with [skip ci] flag

Updated Files

• src/currency_api.rs - New API implementation
• src/lib.rs - Updated doc comments
• src/types/currency.rs - Updated source examples in docs
• src/wasm.rs - Updated test expectations
• tests/integration_test.rs - Updated test assertions
• data/currency/eur-cny.lino - Fixed source attribution
• web/src/App.tsx - Updated footer source text
• web/e2e/calculator.spec.ts - Updated comment
• docs/case-studies/issue-18/README.md - Added note about API change

Test Plan

• All 237 Rust tests pass (cargo test --all-features)
• Clippy passes with no warnings (cargo clippy --all-targets --all-features)
• Formatting check passes (cargo fmt --check)
• File size check passes (node scripts/check-file-size.mjs)
• CI pipeline validates changes
• Manual testing of currency conversion in web UI

Notes

• The ECB does not provide RUB rates, so RUB continues to be sourced from CBR
• Historical rates in .lino files already use ECB/CBR sources
• The weekly cron job will keep rates up-to-date automatically

Fixes #43

🤖 Generated with Claude Code

[konard]

🤖 Solution Draft Log

This log file contains the complete execution trace of the AI solution draft process.

💰 Cost estimation:

• Public pricing estimate: $7.333646 USD
• Calculated by Anthropic: $4.788208 USD
• Difference: $-2.545438 (-34.71%)
  📎 Log file uploaded as Gist (938KB)
  🔗 View complete solution draft log

---

Now working session is ended, feel free to review and add any feedback on the solution draft.