refactor(documents): extract document form sub-components

Shared (src/lib/components/document/):
- WhoWhenSection.svelte: date/location/sender/receivers; owns date state
- DescriptionSection.svelte: title/archive-loc/tags/summary; owns tag binding
- TranscriptionSection.svelte: transcription textarea

Page-local:
- documents/[id]/edit/FileSectionEdit.svelte: current file + replace input
- documents/[id]/edit/SaveBar.svelte: sticky bar with two-step delete confirm
- documents/new/FileSectionNew.svelte: initial file upload input

documents/[id]/edit drops from 319 → ~40 lines.
documents/new drops from 254 → ~30 lines.
Date handling imported from \$lib/utils/date.

Part of #75

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

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

@@ -0,0 +1,80 @@
+<script lang="ts">
+import TagInput from '$lib/components/TagInput.svelte';
+import { m } from '$lib/paraglide/messages.js';
+
+let {
+	tags = $bindable<string[]>([]),
+	initialTitle = '',
+	initialDocumentLocation = '',
+	initialSummary = '',
+	titleRequired = false
+}: {
+	tags?: string[];
+	initialTitle?: string;
+	initialDocumentLocation?: string;
+	initialSummary?: string;
+	titleRequired?: boolean;
+} = $props();
+</script>
+
+<div class="rounded-sm border border-line bg-surface p-6 shadow-sm">
+	<h2 class="mb-5 text-xs font-bold tracking-widest text-ink-3 uppercase">
+		{m.doc_section_description()}
+	</h2>
+
+	<div class="space-y-5">
+		<!-- Titel -->
+		<div>
+			<label for="title" class="mb-1 block text-sm font-medium text-ink-2"
+				>{m.form_label_title()}{#if titleRequired}
+					*{/if}</label
+			>
+			<input
+				id="title"
+				type="text"
+				name="title"
+				value={initialTitle}
+				required={titleRequired}
+				class="block w-full rounded border border-line p-2 text-sm shadow-sm focus:border-ink focus:ring-ink"
+			/>
+		</div>
+
+		<!-- Aufbewahrungsort -->
+		<div>
+			<label for="documentLocation" class="mb-1 block text-sm font-medium text-ink-2"
+				>{m.form_label_archive_location()}</label
+			>
+			<input
+				id="documentLocation"
+				type="text"
+				name="documentLocation"
+				value={initialDocumentLocation}
+				placeholder={m.form_placeholder_archive_location()}
+				class="block w-full rounded border border-line p-2 text-sm shadow-sm focus:border-ink focus:ring-ink"
+			/>
+			<p class="mt-1 text-xs text-ink-3">{m.form_helper_archive_location()}</p>
+		</div>
+
+		<!-- Schlagworte -->
+		<div>
+			<p class="mb-1 block text-sm font-medium text-ink-2">{m.form_label_tags()}</p>
+			<TagInput bind:tags={tags} />
+			<input type="hidden" name="tags" value={tags.join(',')} />
+		</div>
+
+		<!-- Inhalt -->
+		<div>
+			<label for="summary" class="mb-1 block text-sm font-medium text-ink-2"
+				>{m.form_label_content()}</label
+			>
+			<textarea
+				id="summary"
+				name="summary"
+				rows="5"
+				placeholder={m.form_placeholder_content()}
+				class="block w-full rounded border border-line p-2 font-serif text-sm shadow-sm focus:border-ink focus:ring-ink"
+				>{initialSummary}</textarea
+			>
+		</div>
+	</div>
+</div>