feat: multi-account Fio sync + switch QR default to 2502035405/2010

Add second Fio account (CZ0820100000002502035405 / 2502035405/2010).
Both accounts are fetched on every sync run and combined before dedup,
so the payments sheet accumulates transactions from either account.
QR codes now default to the new account.

Go:
- config.go: hardcoded Accounts/LoadedAccount slice replaces scalar
  BankAccount + FioAPIToken; Config.BankAccount renamed QRAccount;
  per-account tokens via FIO_API_TOKEN_NEW / FIO_API_TOKEN_OLD
- banksync.SyncToSheets: accepts []fio.Client, loops to combine txns
- cmd/fuj/main.go: buildFioClients helper; both sync call sites updated
- html_handler + build_adults/juniors: use Config.QRAccount
- New TestSyncToSheets_MultiAccount covers cross-account dedup

Python:
- config.py: ACCOUNTS list + LOADED_ACCOUNTS (tokens from env)
- fio_utils.py: fetch_transactions_for (per-account) +
  fetch_transactions_all (loops all accounts)
- sync_fio_to_sheets.py: uses fetch_transactions_all

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

go/internal/web/html_handler_test.go

@@ -50,7 +50,7 @@ func fixtureHandler(t *testing.T) *api.Handler {
 	t.Helper()
 	return &api.Handler{
 		Sources: fixtureSources{},
-		Config:  config.Config{BankAccount: "CZ0000000000000000000000"},
+		Config:  config.Config{QRAccount: "CZ0000000000000000000000"},
 	}
 }