Fix datetime picker not updating ts_utc before form submission

The datetime picker used only onchange to update the hidden ts_utc field,
but onchange fires on blur, not immediately. On mobile, submitting the form
right after selecting a date would submit before onchange fired, leaving
ts_utc at "0" (defaulting to current time).

Adding oninput ensures the hidden field updates immediately as the value
changes, fixing backdating on all forms using event_datetime_field().

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/animaltrack/web/templates/actions.py

@@ -171,6 +171,7 @@ def event_datetime_field(
                     cls="uk-input w-full mt-2",
                     data_datetime_input=field_id,
                     onchange=f"updateDatetimeTs('{field_id}')",
+                    oninput=f"updateDatetimeTs('{field_id}')",
                 ),
                 P(
                     "Select date/time for this event (leave empty for current time)",