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/services/banksync/sync.go

@@ -35,13 +35,13 @@ type SyncOpts struct {
 	PrintFioTable bool      // with DryRun: print every fetched Fio txn with NEW/DUP status
 }
 
-// SyncToSheets fetches Fio transactions and appends new ones to the payments sheet.
-// Returns the number of rows appended.
+// SyncToSheets fetches Fio transactions from all provided clients and appends
+// new ones to the payments sheet. Returns the number of rows appended.
 // Ports scripts/sync_fio_to_sheets.py sync_to_sheets.
 func SyncToSheets(
 	ctx context.Context,
 	spreadsheetID string,
-	fioClient fio.Client,
+	fioClients []fio.Client,
 	sh sheetsWriter,
 	opts SyncOpts,
 ) (int, error) {
@@ -84,10 +84,14 @@ func SyncToSheets(
 		from = to.AddDate(0, 0, -days)
 	}
 
-	// 3. Fetch Fio transactions.
-	txns, err := fioClient.FetchTransactions(ctx, from, to)
-	if err != nil {
-		return 0, fmt.Errorf("sync: fetch fio: %w", err)
+	// 3. Fetch Fio transactions from each account and combine.
+	var txns []fio.Transaction
+	for _, client := range fioClients {
+		got, err := client.FetchTransactions(ctx, from, to)
+		if err != nil {
+			return 0, fmt.Errorf("sync: fetch fio: %w", err)
+		}
+		txns = append(txns, got...)
 	}
 	if opts.DryRun {
 		fmt.Printf("Dry run: window %s to %s, fetched %d transaction(s) from Fio\n",