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/FileSwitcherStrip.svelte

@@ -4,7 +4,11 @@ import { m } from '$lib/paraglide/messages.js';
 
 export interface FileEntry {
 	id: string;
-	file: File;
+	/** Present in upload mode only. Edit mode entries reference an existing
+	 *  document by `documentId` and have no local file blob. */
+	file?: File;
+	/** Present in edit mode only — the server-side document UUID being edited. */
+	documentId?: string;
 	title: string;
 	status: 'idle' | 'error';
 	previewUrl: string;