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/sync_fio_to_sheets.py

@@ -12,7 +12,7 @@ from google_auth_oauthlib.flow import InstalledAppFlow
 from google.oauth2 import service_account
 from googleapiclient.discovery import build
 
-from fio_utils import fetch_transactions
+from fio_utils import fetch_transactions_all
 
 from config import PAYMENTS_SHEET_ID as DEFAULT_SPREADSHEET_ID
 SCOPES = ["https://www.googleapis.com/auth/spreadsheets"]
@@ -188,7 +188,7 @@ def sync_to_sheets(
         dt_str = date_to.strftime("%Y-%m-%d")
 
     print(f"Fetching Fio transactions from {df_str} to {dt_str}...")
-    transactions = fetch_transactions(df_str, dt_str)
+    transactions = fetch_transactions_all(df_str, dt_str)
     print(f"Found {len(transactions)} transactions.")
 
     if dry_run: