fix(timeline): wire the server date error to the date field's aria-invalid

REQ-011 asks for both required-field errors via per-field aria-invalid, but the
blank-date error was rendered as a standalone <p> after the date card while the
date input's aria-invalid only reflected the client-side malformed-date cue.

DatePrecisionField gains a `dateError` prop: a server error now marks the field
aria-invalid and renders inline under the input (sharing the same error id), and
EventForm drops its detached <p>. While here, migrate the field's two error
texts from hard-coded text-red-600 to the semantic `text-danger` token so they
keep ≥4.5:1 contrast in dark mode (the token remaps; #dc2626 was borderline) —
this also fixes the contrast for WhoWhenSection, the other consumer.

Component test asserts the date input gains aria-invalid on a server date error.

Addresses PR #832 review (Requirements Engineer REQ-011; UI/UX dark-mode contrast).

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

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

@@ -35,6 +35,7 @@ let {
 	suggestedDateIso = '',
 	dateLabel = m.form_label_date(),
 	dateRequired = true,
+	dateError = '',
 	dateTestId = undefined,
 	precisionTestId = undefined,
 	endDateInnerTestId = undefined
@@ -49,6 +50,8 @@ let {
 	suggestedDateIso?: string;
 	dateLabel?: string;
 	dateRequired?: boolean;
+	/** Server-side date error (e.g. blank required field) wired to the field's aria-invalid. */
+	dateError?: string;
 	dateTestId?: string;
 	precisionTestId?: string;
 	endDateInnerTestId?: string;
@@ -83,6 +86,9 @@ onMount(() => {
 });
 
 const dateInvalid = $derived(dateDirty && dateDisplay.length > 0 && dateIso === '');
+// Either the client-side malformed-date cue or a server-provided required-field
+// error marks the field invalid (REQ-011 per-field aria-invalid).
+const dateFieldInvalid = $derived(dateInvalid || dateError.length > 0);
 
 // Inline mirror of the server guard (#678). ISO YYYY-MM-DD strings compare
 // lexicographically, so no Date object is needed. Server stays the gate —
@@ -128,17 +134,21 @@ $effect(() => {
 		maxlength="10"
 		aria-required={dateRequired ? 'true' : undefined}
 		class="block min-h-[48px] w-full rounded border border-line px-2 py-3 text-sm shadow-sm
-               {dateInvalid
+               {dateFieldInvalid
 			? 'border-red-400 focus:outline-none focus-visible:ring-2 focus-visible:ring-red-500'
 			: 'focus:outline-none focus-visible:ring-2 focus-visible:ring-focus-ring'}"
-		aria-invalid={dateInvalid ? 'true' : undefined}
+		aria-invalid={dateFieldInvalid ? 'true' : undefined}
-		aria-describedby={dateInvalid ? `${dateInputName}-error` : undefined}
+		aria-describedby={dateFieldInvalid ? `${dateInputName}-error` : undefined}
 	/>
 	<input type="hidden" name={dateInputName} value={dateIso} />
 	{#if dateInvalid}
-		<p id="{dateInputName}-error" class="mt-1 text-xs text-red-600">
+		<p id="{dateInputName}-error" class="mt-1 text-xs text-danger">
 			<span aria-hidden="true">⚠ </span>{m.form_date_error()}
 		</p>
+	{:else if dateError}
+		<p id="{dateInputName}-error" class="mt-1 text-xs text-danger">
+			<span aria-hidden="true">⚠ </span>{dateError}
+		</p>
 	{/if}
 </div>
 
@@ -183,7 +193,7 @@ $effect(() => {
 			/>
 			{#if endBeforeStart}
 				<!-- Non-colour cue (WCAG 1.4.1): warning glyph + text, not red alone. -->
-				<p id="{dateInputName}-end-error" class="mt-1 text-xs text-red-600">
+				<p id="{dateInputName}-end-error" class="mt-1 text-xs text-danger">
 					<span aria-hidden="true">⚠ </span>{m.error_invalid_date_range()}
 				</p>
 			{/if}

frontend/src/lib/timeline/EventForm.svelte

@@ -221,16 +221,12 @@ async function confirmDelete(e: SubmitEvent) {
 							endDateInputName="eventDateEnd"
 							precisionInputName="precision"
 							dateLabel={m.form_label_date()}
+							dateError={dateError}
 							dateTestId="event-date"
 							precisionTestId="event-precision"
 							endDateInnerTestId="event-end-date"
 						/>
 					</div>
-					{#if dateError}
-						<p class="mt-2 text-sm text-danger" role="alert">
-							<span aria-hidden="true">⚠ </span>{dateError}
-						</p>
-					{/if}
 				</div>
 
 				<!-- Beschreibung -->

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

@@ -73,6 +73,15 @@ describe('EventForm — required-field error (REQ-010)', () => {
 	});
 });
 
+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 () => {
+		render(EventForm, { form: { dateError: 'Bitte ein Datum eingeben.' } });
+		await expect.element(page.getByText('Bitte ein Datum eingeben.')).toBeInTheDocument();
+		const dateInput = document.querySelector('#eventDate') as HTMLInputElement;
+		expect(dateInput.getAttribute('aria-invalid')).toBe('true');
+	});
+});
+
 describe('EventForm — submitting state (named AC)', () => {
 	it('renders an enabled submit button initially', async () => {
 		render(EventForm, { event: makeEvent() });