feat(bulk-upload): guard save() against concurrent invocations

Adds a saving $state flag that blocks re-entry while a chunk upload is
in flight. The UploadSaveBar save button is disabled via a new disabled
prop while saving is true. Tested: clicking Save twice fires fetch only
once.

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

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

@@ -31,6 +31,7 @@ let {
 let files = new SvelteMap<string, FileEntry>();
 let activeId = $state<string | null>(null);
 let chunkProgress = $state<{ done: number; total: number } | undefined>(undefined);
+let saving = $state(false);
 
 // --- Shared metadata ---
 let senderId = $state(untrack(() => initialSenderId));
@@ -84,6 +85,8 @@ onDestroy(() => {
 
 // --- Save ---
 async function save() {
+	if (saving) return;
+	saving = true;
 	const entries = Array.from(files.values());
 	const chunkSize = 10;
 	const chunks: FileEntry[][] = [];
@@ -124,6 +127,7 @@ async function save() {
 		}
 		chunkProgress = { done: i + 1, total: chunks.length };
 	}
+	saving = false;
 	if (!hadErrors) goto('/documents');
 }
 </script>
@@ -276,6 +280,7 @@ async function save() {
 				chunkProgress={chunkProgress}
 				onSave={save}
 				onDiscard={discardAll}
+				disabled={saving}
 			/>
 		</div>
 	</div>