feat(frontend): reorder WhoWhenSection grid, expose dateIso bindable, add autofocus

Required fields (Datum, Absender) move to row 1; optional fields (Empfänger, Ort)
to row 2. dateIso is now bindable for the progress bar. Autofocus lands on the
first empty required field on page load.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/components/document/WhoWhenSection.svelte

@@ -14,6 +14,7 @@ interface Person {
 let {
 	senderId = $bindable(''),
 	selectedReceivers = $bindable<Person[]>([]),
+	dateIso = $bindable(''),
 	initialDateIso = '',
 	initialLocation = '',
 	initialSenderName = '',
@@ -22,6 +23,7 @@ let {
 }: {
 	senderId?: string;
 	selectedReceivers?: Person[];
+	dateIso?: string;
 	initialDateIso?: string;
 	initialLocation?: string;
 	initialSenderName?: string;
@@ -30,7 +32,7 @@ let {
 } = $props();
 
 let dateDisplay = $state(untrack(() => isoToGerman(initialDateIso)));
-let dateIso = $state(untrack(() => initialDateIso));
+dateIso = untrack(() => initialDateIso);
 let dateDirty = $state(false);
 
 const dateInvalid = $derived(dateDirty && dateDisplay.length > 0 && dateIso === '');
@@ -57,7 +59,7 @@ $effect(() => {
 	</h2>
 
 	<div class="grid grid-cols-1 gap-5 md:grid-cols-2">
-		<!-- Datum -->
+		<!-- Datum (required — row 1, col 1) -->
 		<div>
 			<label for="documentDate" class="mb-1 block text-sm font-medium text-ink-2"
 				>{m.form_label_date()}</label
@@ -70,6 +72,7 @@ $effect(() => {
 				oninput={handleDateInput}
 				placeholder={m.form_placeholder_date()}
 				maxlength="10"
+				autofocus={!initialDateIso}
 				class="block w-full rounded border border-line p-2 text-sm shadow-sm
                        {dateInvalid ? '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-describedby={dateInvalid ? 'date-error' : undefined}
@@ -80,7 +83,25 @@ $effect(() => {
 			{/if}
 		</div>
 
-		<!-- Ort -->
+		<!-- Absender (required — row 1, col 2) -->
+		<div>
+			<PersonTypeahead
+				name="senderId"
+				label={m.form_label_sender()}
+				bind:value={senderId}
+				initialName={initialSenderName}
+				suggestedName={suggestedSenderName}
+				autofocus={!!initialDateIso}
+			/>
+		</div>
+
+		<!-- Empfänger (optional — row 2, col 1) -->
+		<div>
+			<p class="mb-1 block text-sm font-medium text-ink-2">{m.form_label_receivers()}</p>
+			<PersonMultiSelect bind:selectedPersons={selectedReceivers} />
+		</div>
+
+		<!-- Ort (optional — row 2, col 2) -->
 		<div>
 			<label for="location" class="mb-1 block text-sm font-medium text-ink-2"
 				>{m.form_label_location()}</label
@@ -94,22 +115,5 @@ $effect(() => {
 				class="block w-full rounded border border-line p-2 text-sm shadow-sm focus:outline-none focus-visible:ring-2 focus-visible:ring-focus-ring"
 			/>
 		</div>
-
-		<!-- Absender -->
-		<div>
-			<PersonTypeahead
-				name="senderId"
-				label={m.form_label_sender()}
-				bind:value={senderId}
-				initialName={initialSenderName}
-				suggestedName={suggestedSenderName}
-			/>
-		</div>
-
-		<!-- Empfänger -->
-		<div>
-			<p class="mb-1 block text-sm font-medium text-ink-2">{m.form_label_receivers()}</p>
-			<PersonMultiSelect bind:selectedPersons={selectedReceivers} />
-		</div>
 	</div>
 </div>