feat(bulk-edit): extend BulkDocumentEditLayout with mode="edit"

- New FieldLabelBadge component (additive / replace variants, WCAG AA contrast)
- WhoWhenSection: hideDate prop, editMode prop renders badges next to sender
  and receivers, hides the meta_location field
- DescriptionSection: editMode prop renders badges next to tags and archive
  fields; new bindable archiveBox / archiveFolder inputs only in editMode
- PersonTypeahead: optional badge prop forwards to FieldLabelBadge
- FileSwitcherStrip FileEntry: file is now optional, documentId added so
  edit-mode entries reference an existing document by UUID
- BulkDocumentEditLayout: mode prop branches drop zone / read-only title /
  callout / save handler. Edit save chunks 500 IDs per PATCH, stops on chunk
  failure with retry, marks per-document errors as chips, clears the bulk
  selection store on full success.

Refs #225

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

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

@@ -0,0 +1,16 @@
+<script lang="ts">
+import { m } from '$lib/paraglide/messages.js';
+
+let { variant }: { variant: 'additive' | 'replace' } = $props();
+
+const text = $derived(
+	variant === 'additive' ? m.bulk_edit_badge_additive() : m.bulk_edit_badge_replace()
+);
+</script>
+
+<span
+	data-testid="field-label-badge-{variant}"
+	class="ml-2 inline-flex items-center rounded-sm bg-muted px-1.5 py-0.5 text-[10px] font-medium tracking-wide text-gray-600"
+>
+	{text}
+</span>