fix: handle >6-digit fractional seconds in ISO timestamps on Windows

[haosenwang1018]

Problem

On Windows, ls fails with:

ValueError: Invalid isoformat string: '2026-02-21T13:20:23.1470042+08:00'

Windows generates ISO timestamps with 7-digit fractional seconds, but Python's datetime.fromisoformat() only supports up to 6 digits.

Fix

Add a parse_iso_datetime() helper in time_utils.py that truncates excess fractional digits before parsing. Replace the two datetime.fromisoformat() calls in viking_fs.py with this helper.

Fixes #242

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