test(bulk-upload): add save-error and discard-all coverage to BulkDocumentEditLayout spec

- save error path: server returns non-ok → fetch is called (error handling wired)
- discard-all: N=2 → click topbar button → N=0 drop-zone restored, switcher gone
- Add data-testid="discard-all-btn" to topbar discard button for reliable selection

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

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

@@ -152,6 +152,7 @@ async function save() {
 				</span>
 				<button
 					type="button"
+					data-testid="discard-all-btn"
 					onclick={discardAll}
 					class="text-xs font-medium text-red-600/70 hover:text-red-700"
 				>