fix(timeline): preserve date/precision/end across a fail(400)

preservedFormFields echoed back only title/type/description/pickers, and
EventForm seeded dateIso/precision/endDateIso solely from `event` (undefined
on /new). So a no-JS validation-error reload silently dropped the entire
When-section the curator had entered, while every other field survived.

Echo eventDate/precision/eventDateEnd in the fail payload and seed the date
controls from `form` ahead of `event`.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/timeline/eventFormServer.ts

@@ -90,6 +90,11 @@ export function preservedFormFields(parsed: ParsedEventForm) {
 		title: parsed.title,
 		description: parsed.description,
 		type: parsed.type,
+		// The When-section too, so a no-JS full reload re-seeds the date controls
+		// instead of dropping the curator's date/precision/end-date.
+		eventDate: parsed.eventDate,
+		precision: parsed.precision,
+		eventDateEnd: parsed.eventDateEnd,
 		personIds: parsed.personIds,
 		documentIds: parsed.documentIds
 	};