fix(bulk-upload): no layout shift, no autofocus on date field

Replace JS navHeight measurement with CSS var(--header-height) so the fixed
panel renders in its final position on first paint — no onMount shift.

Add autofocus prop to WhoWhenSection (default true, preserves document-edit
behaviour) and pass autofocus={false} from BulkDocumentEditLayout so the date
field does not steal focus before the user has even dropped any files.

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

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

@@ -1,7 +1,7 @@
 <script lang="ts">
 import { SvelteMap } from 'svelte/reactivity';
 import { goto } from '$app/navigation';
-import { onMount, onDestroy, untrack } from 'svelte';
+import { onDestroy, untrack } from 'svelte';
 import { m } from '$lib/paraglide/messages.js';
 import BulkDropZone from './BulkDropZone.svelte';
 import FileSwitcherStrip from './FileSwitcherStrip.svelte';
@@ -27,12 +27,6 @@ let {
 	initialReceivers?: Person[];
 } = $props();
 
-// --- Layout ---
-let navHeight = $state(0);
-onMount(() => {
-	navHeight = document.querySelector('header')?.getBoundingClientRect().height ?? 0;
-});
-
 // --- File state ---
 let files = new SvelteMap<string, FileEntry>();
 let activeId = $state<string | null>(null);
@@ -124,7 +118,7 @@ async function save() {
 }
 </script>
 
-<div class="fixed inset-x-0 bottom-0 flex flex-col" style="top: {navHeight}px">
+<div class="fixed inset-x-0 bottom-0 flex flex-col" style="top: var(--header-height)">
 	<!-- Topbar -->
 	<div class="flex shrink-0 items-center gap-3 border-b border-line bg-surface px-6 py-3">
 		<a
@@ -226,6 +220,7 @@ async function save() {
 							bind:selectedReceivers={selectedReceivers}
 							bind:dateIso={dateIso}
 							initialSenderName={initialSenderName}
+							autofocus={false}
 						/>
 						<DescriptionSection bind:tags={tags} hideTitle />
 					</ScopeCard>