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>

legacy/scripts/sync_fio_to_sheets.py

@@ -59,6 +59,16 @@ def get_sheets_service(credentials_path: str):
     return _SHEETS_SERVICE
 
 
+def as_text_cell(value) -> str:
+    """Mark a value to be stored as text when written with USER_ENTERED.
+
+    A leading apostrophe stops Sheets from parsing digits as a number (which
+    would drop leading zeros); it is not part of the stored value. Empty stays empty.
+    """
+    s = "" if value is None else str(value)
+    return f"'{s}" if s else ""
+
+
 def generate_sync_id(tx: dict) -> str:
     """Generate a unique SHA-256 hash for a transaction.
     
@@ -209,7 +219,7 @@ def sync_to_sheets(
                 "", # Purpose
                 "", # Inferred Amount
                 tx.get("sender", ""),
-                tx.get("vs", ""),
+                as_text_cell(tx.get("vs", "")),
                 tx.get("message", ""),
                 tx.get("bank_id", ""),
                 sync_id,