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.spec.ts

@@ -206,6 +206,34 @@ describe('BulkDocumentEditLayout', () => {
 		);
 	});
 
+	it('save() does not call fetch a second time when already saving', async () => {
+		let resolveFirst: (() => void) | undefined;
+		const mockFetch = vi.fn().mockImplementation(
+			() =>
+				new Promise<Response>((resolve) => {
+					resolveFirst = () =>
+						resolve({
+							ok: true,
+							json: async () => ({ created: [], updated: [], errors: [] })
+						} as Response);
+				})
+		);
+		vi.stubGlobal('fetch', mockFetch);
+
+		const { container } = render(BulkDocumentEditLayout, {});
+		await addFilesViaInput(container, [makeFile('a.pdf')]);
+
+		const saveBtn = container.querySelector(
+			'button[data-testid="bulk-save-btn"]'
+		) as HTMLButtonElement;
+		saveBtn.click(); // first click — fetch is in-flight
+		saveBtn.click(); // second click — should be a no-op
+
+		resolveFirst?.();
+		await vi.waitFor(() => expect(mockFetch).toHaveBeenCalledTimes(1), { timeout: 3000 });
+		expect(mockFetch).toHaveBeenCalledTimes(1);
+	});
+
 	it('discard-all resets to N=0 state and shows drop zone', async () => {
 		const { container } = render(BulkDocumentEditLayout, {});
 		await addFilesViaInput(container, [makeFile('a.pdf'), makeFile('b.pdf')]);