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/routes/documents/new/FileSectionNew.svelte

@@ -1,5 +1,17 @@
 <script lang="ts">
 import { m } from '$lib/paraglide/messages.js';
+import { parseFilename, type FilenameParseResult } from '$lib/utils/filename';
+
+let {
+	onfileParsed
+}: {
+	onfileParsed?: (result: FilenameParseResult) => void;
+} = $props();
+
+function handleFileChange(e: Event) {
+	const file = (e.target as HTMLInputElement).files?.[0];
+	if (file) onfileParsed?.(parseFilename(file.name));
+}
 </script>
 
 <div class="rounded-sm border border-line bg-surface p-6 shadow-sm">
@@ -15,6 +27,7 @@ import { m } from '$lib/paraglide/messages.js';
 		id="file-upload"
 		type="file"
 		name="file"
+		onchange={handleFileChange}
 		class="block w-full cursor-pointer text-sm
                text-ink-2 file:mr-4 file:rounded
                file:border-0 file:bg-muted