feat(frontend): wire progress bar, upload zone, and file replace into enrich page

- Required-fields progress bar (Pflichtfelder) with role="progressbar" ARIA tracks
  Titel, Datum, and Absender live via bound props from child components
- Left panel shows UploadZone for PLACEHOLDER documents (no filePath); after upload
  invalidates 'app:document' to transition to PDF viewer without page reload
- AbortController powers the cancel button during upload
- "Datei ersetzen" ghost button lives in a thin toolbar above the PDF viewer
- dateIso and currentTitle are now bound from WhoWhenSection/DescriptionSection

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

frontend/src/routes/enrich/[id]/+page.svelte

@@ -1,9 +1,12 @@
 <script lang="ts">
 import { enhance } from '$app/forms';
+import { invalidate } from '$app/navigation';
 import { onMount, onDestroy, untrack } from 'svelte';
 import { createFileLoader } from '$lib/hooks/useFileLoader.svelte';
 import { m } from '$lib/paraglide/messages.js';
+import { countRequiredFilled } from '$lib/utils/requiredFields';
 import DocumentViewer from '$lib/components/DocumentViewer.svelte';
+import UploadZone from '$lib/components/document/UploadZone.svelte';
 import WhoWhenSection from '$lib/components/document/WhoWhenSection.svelte';
 import DescriptionSection from '$lib/components/document/DescriptionSection.svelte';
 
@@ -36,6 +39,54 @@ onDestroy(() => fileLoader.destroy());
 let tags = $state(untrack(() => doc.tags ?? []));
 let senderId = $state(untrack(() => doc.sender?.id ?? ''));
 let selectedReceivers = $state(untrack(() => doc.receivers ?? []));
+
+// Progress bar state — bound from child components
+let dateIso = $state(untrack(() => doc.documentDate ?? ''));
+let currentTitle = $state(untrack(() => doc.title ?? ''));
+
+const requiredFilled = $derived(countRequiredFilled(currentTitle, dateIso, senderId));
+const requiredPct = $derived((requiredFilled / 3) * 100);
+
+// Upload state machine
+let isUploading = $state(false);
+let isDragging = $state(false);
+let uploadError = $state<string | null>(null);
+let abortController = $state<AbortController | null>(null);
+
+async function handleFile(file: File) {
+	uploadError = null;
+	isUploading = true;
+	const controller = new AbortController();
+	abortController = controller;
+	try {
+		const formData = new FormData();
+		formData.append('file', file);
+		const res = await fetch(`/api/documents/${doc.id}/file`, {
+			method: 'POST',
+			body: formData,
+			signal: controller.signal
+		});
+		if (!res.ok) throw new Error('Upload fehlgeschlagen');
+		await invalidate('app:document');
+	} catch (e) {
+		if ((e as Error).name === 'AbortError') return;
+		uploadError = 'Upload fehlgeschlagen. Bitte erneut versuchen.';
+	} finally {
+		isUploading = false;
+		abortController = null;
+	}
+}
+
+function cancelUpload() {
+	abortController?.abort();
+	isUploading = false;
+}
+
+async function handleReplaceFile(e: Event) {
+	const file = (e.currentTarget as HTMLInputElement).files?.[0];
+	if (!file) return;
+	await handleFile(file);
+}
 </script>
 
 <svelte:head>
@@ -67,20 +118,61 @@ let selectedReceivers = $state(untrack(() => doc.receivers ?? []));
 		</p>
 	</div>
 
+	<!-- Required-fields progress bar -->
+	<div class="flex items-center gap-3 border-b border-line bg-surface px-6 py-1.5">
+		<span class="text-[9px] font-bold tracking-widest text-ink-3 uppercase">Pflichtfelder</span>
+		<div
+			class="h-0.5 flex-1 rounded-full bg-line"
+			role="progressbar"
+			aria-valuenow={requiredFilled}
+			aria-valuemin={0}
+			aria-valuemax={3}
+			aria-label="Pflichtfelder"
+		>
+			<div
+				class="h-full rounded-full bg-brand-navy transition-all duration-300"
+				style="width:{requiredPct}%"
+			></div>
+		</div>
+		<span class="text-[10px] font-bold text-brand-navy">{requiredFilled} / 3</span>
+	</div>
+
 	<!-- Main content -->
 	<div class="flex flex-1 overflow-hidden">
-		<!-- Left: PDF preview (60%) -->
-		<div class="relative flex-[6] overflow-hidden border-r border-line">
-			<DocumentViewer
-				doc={doc}
-				fileUrl={fileLoader.fileUrl}
-				isLoading={fileLoader.isLoading}
-				error={fileLoader.fileError}
-				bind:annotateMode={annotateMode}
-				bind:activeAnnotationId={activeAnnotationId}
-				bind:activeAnnotationPage={activeAnnotationPage}
-				onAnnotationClick={() => {}}
-			/>
+		<!-- Left: PDF preview / upload zone (60%) -->
+		<div class="relative flex flex-[6] flex-col overflow-hidden border-r border-line">
+			{#if !doc.filePath}
+				<UploadZone
+					filename={doc.originalFilename}
+					isUploading={isUploading}
+					bind:isDragging={isDragging}
+					error={uploadError}
+					onFile={handleFile}
+					onCancel={cancelUpload}
+				/>
+			{:else}
+				<!-- Datei ersetzen toolbar (ghost button above PDF) -->
+				<div class="flex shrink-0 items-center border-b border-white/10 bg-pdf-bg px-4 py-1.5">
+					<label
+						class="ml-auto flex min-h-[44px] cursor-pointer items-center text-xs font-bold tracking-widest text-white/40 uppercase transition-colors hover:text-white/70"
+					>
+						Datei ersetzen
+						<input type="file" class="sr-only" onchange={handleReplaceFile} />
+					</label>
+				</div>
+				<div class="relative flex-1 overflow-hidden">
+					<DocumentViewer
+						doc={doc}
+						fileUrl={fileLoader.fileUrl}
+						isLoading={fileLoader.isLoading}
+						error={fileLoader.fileError}
+						bind:annotateMode={annotateMode}
+						bind:activeAnnotationId={activeAnnotationId}
+						bind:activeAnnotationPage={activeAnnotationPage}
+						onAnnotationClick={() => {}}
+					/>
+				</div>
+			{/if}
 		</div>
 
 		<!-- Right: form (40%) -->
@@ -102,13 +194,17 @@ let selectedReceivers = $state(untrack(() => doc.receivers ?? []));
 				<WhoWhenSection
 					bind:senderId={senderId}
 					bind:selectedReceivers={selectedReceivers}
+					bind:dateIso={dateIso}
 					initialDateIso={doc.documentDate ?? ''}
 					initialLocation={doc.location ?? ''}
-					initialSenderName={doc.sender
-						? doc.sender.displayName
-						: ''}
+					initialSenderName={doc.sender ? doc.sender.displayName : ''}
+				/>
+				<DescriptionSection
+					bind:tags={tags}
+					bind:currentTitle={currentTitle}
+					initialTitle={doc.title ?? ''}
+					titleRequired={true}
 				/>
-				<DescriptionSection bind:tags={tags} initialTitle={doc.title ?? ''} titleRequired={true} />
 			</form>
 
 			<!-- Skip form (outside main form to avoid nesting) -->