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/[id]/edit/SaveBar.svelte

@@ -0,0 +1,72 @@
+<script lang="ts">
+import { m } from '$lib/paraglide/messages.js';
+
+let { docId }: { docId: string } = $props();
+
+let confirmDelete = $state(false);
+</script>
+
+<div
+	class="sticky bottom-0 z-10 -mx-4 flex items-center justify-between border-t border-line bg-surface px-6 py-4 shadow-[0_-2px_8px_rgba(0,0,0,0.06)]"
+>
+	<!-- Left: delete -->
+	<div class="flex items-center gap-3">
+		{#if confirmDelete}
+			<span class="font-sans text-sm text-red-700">{m.doc_delete_confirm()}</span>
+			<button
+				type="submit"
+				form="delete-form"
+				class="rounded bg-red-600 px-4 py-1.5 text-sm font-bold text-white transition-colors hover:bg-red-700"
+			>
+				{m.btn_delete()}
+			</button>
+			<button
+				type="button"
+				onclick={() => (confirmDelete = false)}
+				class="text-sm text-ink-2 transition-colors hover:text-ink"
+			>
+				{m.btn_cancel()}
+			</button>
+		{:else}
+			<button
+				type="button"
+				onclick={() => (confirmDelete = true)}
+				class="flex items-center gap-1.5 rounded border border-red-300 px-4 py-1.5 text-sm font-bold text-red-600 transition-colors hover:border-red-600 hover:bg-red-50"
+			>
+				<svg
+					xmlns="http://www.w3.org/2000/svg"
+					class="h-4 w-4"
+					viewBox="0 0 24 24"
+					fill="none"
+					stroke="currentColor"
+					stroke-width="2"
+					stroke-linecap="round"
+					stroke-linejoin="round"
+					aria-hidden="true"
+				>
+					<polyline points="3 6 5 6 21 6" />
+					<path d="M19 6l-1 14a2 2 0 0 1-2 2H8a2 2 0 0 1-2-2L5 6" />
+					<path d="M10 11v6M14 11v6" />
+					<path d="M9 6V4a1 1 0 0 1 1-1h4a1 1 0 0 1 1 1v2" />
+				</svg>
+				{m.btn_delete()}
+			</button>
+		{/if}
+	</div>
+
+	<!-- Right: cancel + save -->
+	<div class="flex items-center gap-4">
+		<a
+			href="/documents/{docId}"
+			class="text-sm font-medium text-ink-2 transition-colors hover:text-ink"
+		>
+			{m.btn_cancel()}
+		</a>
+		<button
+			type="submit"
+			class="rounded bg-primary px-6 py-2 text-sm font-bold tracking-widest text-white uppercase transition-colors hover:bg-primary/80"
+		>
+			{m.btn_save()}
+		</button>
+	</div>
+</div>