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/WhoWhenSection.svelte

@@ -16,7 +16,8 @@ let {
 	initialLocation = '',
 	initialSenderName = '',
 	suggestedDateIso = '',
-	suggestedSenderName = ''
+	suggestedSenderName = '',
+	autofocus = true
 }: {
 	senderId?: string;
 	selectedReceivers?: Person[];
@@ -26,6 +27,7 @@ let {
 	initialSenderName?: string;
 	suggestedDateIso?: string;
 	suggestedSenderName?: string;
+	autofocus?: boolean;
 } = $props();
 
 let dateDisplay = $state(untrack(() => isoToGerman(initialDateIso)));
@@ -69,7 +71,7 @@ $effect(() => {
 				oninput={handleDateInput}
 				placeholder={m.form_placeholder_date()}
 				maxlength="10"
-				autofocus={!initialDateIso}
+				autofocus={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}
@@ -89,7 +91,7 @@ $effect(() => {
 				bind:value={senderId}
 				initialName={initialSenderName}
 				suggestedName={suggestedSenderName}
-				autofocus={!!initialDateIso}
+				autofocus={autofocus && !!initialDateIso}
 			/>
 		</div>