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>

scripts/config.py

@@ -21,8 +21,18 @@ PAYMENTS_SHEET_ID = "1Om0YPoDVCH5cV8BrNz5LG5eR5MMU05ypQC7UMN1xn_Y"
 # Attendance sheet tab GIDs
 JUNIOR_SHEET_GID = "1213318614"
 
-# Bank
-BANK_ACCOUNT = os.environ.get("BANK_ACCOUNT", "CZ8520100000002800359168")
+# Bank accounts — hardcoded list mirroring go/internal/config/config.go.
+# The entry with primary=True is used for QR codes and as the BANK_ACCOUNT default.
+ACCOUNTS = [
+    {"iban": "CZ0820100000002502035405", "acct_num": "2502035405", "token_env": "FIO_API_TOKEN_NEW", "primary": True},
+    {"iban": "CZ8520100000002800359168", "acct_num": "2800359168", "token_env": "FIO_API_TOKEN_OLD", "primary": False},
+]
+# Resolve API tokens from the environment once at import time.
+LOADED_ACCOUNTS = [
+    {**a, "token": os.environ.get(a["token_env"], "")}
+    for a in ACCOUNTS
+]
+BANK_ACCOUNT = next(a["iban"] for a in ACCOUNTS if a["primary"])
 
 # Cache settings
 CACHE_DIR = PROJECT_ROOT / "tmp"