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/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) {