feat(new-doc): pre-fill date, sender and title from parsed filename

When a file is selected on the new document page, parseFilename runs
on the filename and suggests date, sender name and title via the new
suggestedDateIso / suggestedSenderName / suggestedTitle props. Each
suggestion is applied only while the respective field is still clean
(not dirty), so manual input is never overwritten.

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

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

@@ -16,13 +16,17 @@ let {
 	selectedReceivers = $bindable<Person[]>([]),
 	initialDateIso = '',
 	initialLocation = '',
-	initialSenderName = ''
+	initialSenderName = '',
+	suggestedDateIso = '',
+	suggestedSenderName = ''
 }: {
 	senderId?: string;
 	selectedReceivers?: Person[];
 	initialDateIso?: string;
 	initialLocation?: string;
 	initialSenderName?: string;
+	suggestedDateIso?: string;
+	suggestedSenderName?: string;
 } = $props();
 
 let dateDisplay = $state(untrack(() => isoToGerman(initialDateIso)));
@@ -37,6 +41,14 @@ function handleDateInput(e: Event) {
 	dateIso = result.iso;
 	dateDirty = true;
 }
+
+$effect(() => {
+	const suggested = suggestedDateIso;
+	if (suggested && !untrack(() => dateDirty)) {
+		dateDisplay = isoToGerman(suggested);
+		dateIso = suggested;
+	}
+});
 </script>
 
 <div class="rounded-sm border border-line bg-surface p-6 shadow-sm">
@@ -90,6 +102,7 @@ function handleDateInput(e: Event) {
 				label={m.form_label_sender()}
 				bind:value={senderId}
 				initialName={initialSenderName}
+				suggestedName={suggestedSenderName}
 			/>
 		</div>