Smart Search produced takenAfter/takenBefore as plain YYYY-MM-DD strings,
but the Immich search API validates them as ISO 8601 datetimes and rejects
a bare date with invalid_format / format: datetime. Every search containing
a date failed with 'Failed to fetch assets'.
Expand date-only values after the existing today-guardrail (which compares
YYYY-MM-DD strings and must keep doing so): takenAfter to the start of the
day, takenBefore to the end of the day so a single-day range such as
'videos yesterday' is not empty. Values that already carry a time component
are left untouched.
