fix: treat VS as a string on read and write

Go stringified sheet cells with fmt.Sprint, so a numeric VS read with
UNFORMATTED_VALUE was rendered as 2.609767878e+09. Text columns of the
payments sheet now go through sheets.CellString, which never uses
exponent form; bank inference uses the same helper.

Both sync implementations now append VS as a text cell so Sheets keeps
it a string and leading zeros survive. Existing rows are not rewritten.

Python already read VS as a digit string; a regression test pins that.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

go/internal/services/banksync/infer.go

@@ -94,7 +94,7 @@ func InferPayments(
 			if idx < 0 || idx >= len(row) {
 				return ""
 			}
-			return strings.TrimSpace(fmt.Sprint(row[idx]))
+			return strings.TrimSpace(sheets.CellString(row[idx]))
 		}
 
 		// Skip rule: any of manual fix / Person / Purpose non-empty → leave alone

go/internal/services/banksync/sync.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"fuj-management/go/internal/domain/synch"
 	"fuj-management/go/internal/io/fio"
+	"fuj-management/go/internal/io/sheets"
 	"os"
 	"strings"
 	"time"
@@ -130,7 +131,7 @@ func SyncToSheets(
 		newRows = append(newRows, []any{
 			tx.Date, tx.Amount,
 			"", "", "", "", // manual fix, Person, Purpose, Inferred Amount
-			tx.Sender, tx.VS, tx.Message, tx.BankID, syncIDs[i],
+			tx.Sender, sheets.TextCell(tx.VS), tx.Message, tx.BankID, syncIDs[i],
 		})
 	}
 
@@ -141,7 +142,7 @@ func SyncToSheets(
 	if opts.DryRun {
 		for _, row := range newRows {
 			fmt.Printf("Dry run: would append date=%v amount=%v sender=%v vs=%v message=%v\n",
-				row[0], row[1], row[6], row[7], row[8])
+				row[0], row[1], row[6], strings.TrimPrefix(fmt.Sprint(row[7]), "'"), row[8])
 		}
 		if opts.Sort {
 			fmt.Println("Dry run: would sort by date")

go/internal/services/banksync/sync_test.go

@@ -38,6 +38,10 @@ func TestSyncToSheets_EmptySheet(t *testing.T) {
 	if syncID, ok := rows[0][10].(string); !ok || len(syncID) != 64 {
 		t.Errorf("expected 64-char hex sync ID, got %v", rows[0][10])
 	}
+	// VS (index 7) is written as text so Sheets keeps it a string.
+	if vs, _ := rows[0][7].(string); vs != "'123" {
+		t.Errorf("expected VS written as text cell '123, got %v", rows[0][7])
+	}
 }
 
 func TestSyncToSheets_Dedup(t *testing.T) {