Handle qcodes timestamp timezone format change and render timestamps in local time

[Copilot]

Upcoming qcodes changes the default render format of run_timestamp()/completed_timestamp() (and raw_time_to_str_time) from "%Y-%m-%d %H:%M:%S" to "%Y-%m-%d %H:%M:%S%z", so timestamps now carry the local UTC offset (e.g. 2026-07-31 10:27:25+0200). This audits where plottr consumes those strings and ensures rendered timestamps use the machine's local timezone.

Changes

• _split_timestamp (plottr/data/qcodes_dataset.py): converts timezone-aware timestamps to the local (PC) timezone via datetime.astimezone() before formatting, drops the offset from the split date/time, and handles both legacy and %z formats; returns ('', '') on any parse/convert error.
• test_get_ds_info (test/pytest/test_qcodes_data.py): previously sliced the raw string (ts[11:]), which now includes +0200 and mismatches get_ds_info(). Reuses _split_timestamp to build expected values.

Notes on impact

• The plot info panel (Started:/Finished:) passes the raw qcodes string through, so it will now display the offset suffix — accurate and already local; left unchanged.
• _qcodes_db_overview.py::_format_timestamp renders from raw unix floats via datetime.fromtimestamp (local time) and is unaffected.

# aware input rendered in the PC's local timezone (UTC here); legacy input preserved
_split_timestamp("2026-07-31 10:27:25+0200")  # -> ("2026-07-31", "08:27:25")
_split_timestamp("2026-07-31 10:27:25")       # -> ("2026-07-31", "10:27:25")