feat: add transactions export command

[johann-taberlet]

Summary

Adds a new transactions subcommand to export account transactions over a date range, using BoursoBank's CSV export endpoint (/budget/exporter-mouvements).

# CSV output (default)
bourso transactions -a <account_id> -f "01/01/2026" -t "31/01/2026"

# JSON output
bourso transactions -a <account_id> -f "01/01/2026" -t "31/01/2026" -o json

Changes

• bourso_api::account: Add Transaction struct with all fields from BoursoBank CSV export
• bourso_api::client::transaction: Add get_transactions() method on BoursoWebClient
  • Hits the /budget/exporter-mouvements endpoint with date range and account filter
  • Handles the 302 redirect to api.boursobank.com for file download (since the client uses Policy::none())
  • Parses BoursoBank CSV format (semicolon-separated, French number format with comma decimals and space thousands separators)
  • Returns empty Vec gracefully when an HTML error page is returned
• CLI: Add transactions subcommand with --account, --from, --to, --output args
• New dependency: csv crate for robust CSV parsing

Test plan

• cargo build --release compiles without errors
• cargo test -p bourso_api — 11 tests pass (3 new: test_parse_amount, test_extract_transactions, test_extract_transactions_empty_html)
• Manual test: successfully exported 8 transactions from a live BoursoBank account with CSV output
• Manual test: JSON output (-o json) works correctly