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/UploadSaveBar.svelte

@@ -5,12 +5,14 @@ let {
 	fileCount,
 	chunkProgress,
 	onSave,
-	onDiscard
+	onDiscard,
+	disabled = false
 }: {
 	fileCount: number;
 	chunkProgress?: { done: number; total: number };
 	onSave: () => void;
-	onDiscard: () => void;
+	onDiscard: () => void | Promise<void>;
+	disabled?: boolean;
 } = $props();
 </script>
 
@@ -37,7 +39,7 @@ let {
 		<button
 			type="button"
 			data-testid="bulk-save-btn"
-			disabled={fileCount === 0}
+			disabled={fileCount === 0 || disabled}
 			onclick={onSave}
 			class="min-h-[44px] rounded-sm bg-primary px-6 text-sm font-bold tracking-widest text-primary-fg uppercase transition-opacity hover:opacity-90 disabled:opacity-40"
 		>