fix(timeline): validate RANGE end-date client-side with a field-level error

A RANGE event with a blank end date passed validateEventForm and reached
the backend, which 400s with a generic INVALID_DATE_RANGE mapped to "end
must not be before start" — wrong for a missing end date, and shown only as
a top-of-form alert. Validate it before the API call and surface a dedicated
event_editor_end_date_required message on the end-date field via a new
DatePrecisionField endDateError prop (defaults '', so the document form is
unchanged).

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

frontend/src/lib/shared/primitives/DatePrecisionField.svelte

@@ -36,6 +36,7 @@ let {
 	dateLabel = m.form_label_date(),
 	dateRequired = true,
 	dateError = '',
+	endDateError = '',
 	onchange = undefined,
 	dateTestId = undefined,
 	precisionTestId = undefined,
@@ -53,6 +54,8 @@ let {
 	dateRequired?: boolean;
 	/** Server-side date error (e.g. blank required field) wired to the field's aria-invalid. */
 	dateError?: string;
+	/** Server-side end-date error (e.g. RANGE without an end date) wired to the end field. */
+	endDateError?: string;
 	/** Called on any user edit (date, precision, end-date) — lets a parent track dirtiness. */
 	onchange?: () => void;
 	dateTestId?: string;
@@ -99,6 +102,9 @@ const dateFieldInvalid = $derived(dateInvalid || dateError.length > 0);
 const endBeforeStart = $derived(
 	showEndDate && endDateIso !== '' && dateIso !== '' && endDateIso < dateIso
 );
+// Either the inline end-before-start cue or a server-provided end-date error
+// (e.g. a RANGE event missing its end date) marks the end field invalid.
+const endDateFieldInvalid = $derived(endBeforeStart || endDateError.length > 0);
 
 function handleDateInput(e: Event) {
 	const result = handleGermanDateInput(e);
@@ -190,10 +196,10 @@ $effect(() => {
 				oninput={handleEndDateInput}
 				placeholder={m.form_placeholder_date()}
 				maxlength="10"
-				aria-invalid={endBeforeStart ? 'true' : undefined}
-				aria-describedby={endBeforeStart ? `${dateInputName}-end-error` : undefined}
+				aria-invalid={endDateFieldInvalid ? 'true' : undefined}
+				aria-describedby={endDateFieldInvalid ? `${dateInputName}-end-error` : undefined}
 				class="block min-h-[48px] w-full rounded border border-line px-2 py-3 text-sm shadow-sm
-               {endBeforeStart
+               {endDateFieldInvalid
 					? 'border-red-400 focus:outline-none focus-visible:ring-2 focus-visible:ring-red-500'
 					: 'focus:outline-none focus-visible:ring-2 focus-visible:ring-focus-ring'}"
 			/>
@@ -202,6 +208,10 @@ $effect(() => {
 				<p id="{dateInputName}-end-error" class="mt-1 text-xs text-danger">
 					<span aria-hidden="true">⚠ </span>{m.error_invalid_date_range()}
 				</p>
+			{:else if endDateError}
+				<p id="{dateInputName}-end-error" class="mt-1 text-xs text-danger">
+					<span aria-hidden="true">⚠ </span>{endDateError}
+				</p>
 			{/if}
 		</div>
 	{/if}

frontend/src/lib/timeline/EventForm.svelte

@@ -25,6 +25,7 @@ interface FormResult {
 	error?: string;
 	titleError?: string;
 	dateError?: string;
+	endDateError?: string;
 	title?: string;
 	description?: string;
 	type?: string;
@@ -104,6 +105,9 @@ const titleError = $derived(
 	titleEmpty && (titleTouched || !!form?.titleError) ? m.event_editor_title_required() : ''
 );
 const dateError = $derived(dateIso ? '' : (form?.dateError ?? ''));
+// Only meaningful for RANGE; clears as soon as an end date is entered. The
+// end-date field is hidden off-RANGE, so a stale value never renders.
+const endDateError = $derived(endDateIso ? '' : (form?.endDateError ?? ''));
 
 beforeNavigate(({ cancel }) => {
 	if (dirty && !submitting) {
@@ -224,6 +228,7 @@ function markDirty() {
 							precisionInputName="precision"
 							dateLabel={m.form_label_date()}
 							dateError={dateError}
+							endDateError={endDateError}
 							onchange={markDirty}
 							dateTestId="event-date"
 							precisionTestId="event-precision"

frontend/src/lib/timeline/EventForm.svelte.spec.ts

@@ -148,6 +148,21 @@ describe('EventForm — seeds the When-section from the fail payload (review #2
 	});
 });
 
+describe('EventForm — RANGE end-date required error wired per-field (review #3)', () => {
+	it('shows the end-date required message on the end-date field, marked invalid', async () => {
+		renderForm({
+			form: {
+				precision: 'RANGE',
+				eventDate: '1925-04-01',
+				endDateError: 'Bitte ein Enddatum eingeben.'
+			}
+		});
+		await expect.element(page.getByText('Bitte ein Enddatum eingeben.')).toBeInTheDocument();
+		const endInput = document.querySelector('#eventDateEnd') as HTMLInputElement;
+		expect(endInput.getAttribute('aria-invalid')).toBe('true');
+	});
+});
+
 describe('EventForm — server date error wired per-field (REQ-011)', () => {
 	it('marks the date field aria-invalid and shows the message on a server date error', async () => {
 		renderForm({ form: { dateError: 'Bitte ein Datum eingeben.' } });

frontend/src/lib/timeline/eventFormServer.ts

@@ -71,17 +71,23 @@ export function parseEventForm(formData: FormData): ParsedEventForm {
 }
 
 /**
- * Returns both failing required-field errors (title + date) simultaneously, or
- * null when the form is valid. The route owns the `fail(400)` so it can enrich
- * the payload with the preserved field values and rehydrated picker selections.
+ * Returns the failing required-field errors (title + date + RANGE end-date)
+ * simultaneously, or null when the form is valid. The route owns the `fail(400)`
+ * so it can enrich the payload with the preserved field values and rehydrated
+ * picker selections.
  */
 export function validateEventForm(
 	parsed: ParsedEventForm
-): { titleError: string; dateError: string } | null {
+): { titleError: string; dateError: string; endDateError: string } | null {
 	const titleError = parsed.title.length === 0 ? m.event_editor_title_required() : '';
 	const dateError = parsed.eventDate.length === 0 ? m.event_editor_date_required() : '';
-	if (!titleError && !dateError) return null;
-	return { titleError, dateError };
+	// A RANGE event requires an end date. Catch it here so it never reaches the
+	// backend, which rejects with a generic INVALID_DATE_RANGE mapped to the wrong
+	// "end before start" message and no field-level cue.
+	const endDateError =
+		parsed.precision === 'RANGE' && !parsed.eventDateEnd ? m.event_editor_end_date_required() : '';
+	if (!titleError && !dateError && !endDateError) return null;
+	return { titleError, dateError, endDateError };
 }
 
 /** The entered field values echoed back in every `fail(...)` so the form re-renders without loss. */

frontend/src/routes/zeitstrahl/events/new/page.server.spec.ts

@@ -208,6 +208,27 @@ describe('zeitstrahl/events/new save action (REQ-004/009/010/015)', () => {
 		expect(failData(result).eventDateEnd).toBe('1944-03-14');
 	});
 
+	it('rejects a RANGE event with no end date before calling the API (review #3)', async () => {
+		// Without this guard the body reaches the backend, which 400s with a generic
+		// INVALID_DATE_RANGE mapped to the wrong "end before start" message.
+		const post = vi.fn();
+		vi.mocked(createApiClient).mockReturnValue({ POST: post, GET: vi.fn() } as never);
+
+		const result = await actions.save(
+			saveEvent({
+				title: 'Umzug',
+				type: 'PERSONAL',
+				eventDate: '1925-04-01',
+				precision: 'RANGE'
+				// eventDateEnd intentionally omitted
+			})
+		);
+
+		expect(post).not.toHaveBeenCalled();
+		expect(result).toMatchObject({ status: 400 });
+		expect(failData(result).endDateError).toBeTruthy();
+	});
+
 	it('surfaces both title and date errors when both blank (REQ-011)', async () => {
 		vi.mocked(createApiClient).mockReturnValue({ POST: vi.fn() } as never);
 		const result = await actions.save(saveEvent({ title: '', type: 'PERSONAL', eventDate: '' }));