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/routes/documents/new/FileSectionNew.svelte

@@ -0,0 +1,25 @@
+<script lang="ts">
+import { m } from '$lib/paraglide/messages.js';
+</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_file()}
+	</h2>
+
+	<label for="file-upload" class="mb-1 block text-sm font-medium text-ink-2">
+		{m.doc_file_upload_label()}
+		<span class="font-normal text-ink-3">({m.doc_file_upload_note()})</span>
+	</label>
+	<input
+		id="file-upload"
+		type="file"
+		name="file"
+		class="block w-full cursor-pointer text-sm
+               text-ink-2 file:mr-4 file:rounded
+               file:border-0 file:bg-muted
+               file:px-4 file:py-2
+               file:text-sm file:font-semibold
+               file:text-ink hover:file:bg-muted"
+	/>
+</div>