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/TranscriptionSection.svelte

@@ -0,0 +1,19 @@
+<script lang="ts">
+import { m } from '$lib/paraglide/messages.js';
+
+let { initialTranscription = '' }: { initialTranscription?: string } = $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.form_label_transcription()}
+	</h2>
+	<textarea
+		id="transcription"
+		name="transcription"
+		rows="12"
+		placeholder={m.form_placeholder_transcription()}
+		class="block w-full rounded border border-line p-2 font-serif text-sm shadow-sm focus:border-ink focus:ring-ink"
+		>{initialTranscription}</textarea
+	>
+</div>