refactor(autosave): rename flushViaBeacon → flushOnUnload; add void to fire-and-forget fetch

The sendBeacon name was misleading after switching to keepalive fetch.
Also adds a test to confirm flush is a no-op when pendingTexts is empty.

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

frontend/src/lib/components/TranscriptionEditView.svelte

@@ -73,7 +73,7 @@ $effect(() => {
 
 $effect(() => {
 	function onBeforeUnload() {
-		autoSave.flushViaBeacon();
+		autoSave.flushOnUnload();
 	}
 	window.addEventListener('beforeunload', onBeforeUnload);
 	return () => {

frontend/src/lib/hooks/__tests__/useBlockAutoSave.svelte.test.ts

@@ -83,7 +83,7 @@ describe('createBlockAutoSave', () => {
 	});
 });
 
-describe('flushViaBeacon', () => {
+describe('flushOnUnload', () => {
 	let mockFetch: Mock;
 
 	beforeEach(() => {
@@ -103,7 +103,7 @@ describe('flushViaBeacon', () => {
 		const as = createBlockAutoSave({ saveFn: mockSaveFn, documentId: 'doc-1' });
 		as.handleTextChange('block-1', 'hello');
 		as.handleTextChange('block-2', 'world');
-		as.flushViaBeacon();
+		as.flushOnUnload();
 
 		expect(mockFetch).toHaveBeenCalledTimes(2);
 		expect(mockFetch).toHaveBeenCalledWith(
@@ -128,14 +128,14 @@ describe('flushViaBeacon', () => {
 		const sendBeaconSpy = vi.spyOn(navigator, 'sendBeacon').mockReturnValue(true);
 		const as = createBlockAutoSave({ saveFn: mockSaveFn, documentId: 'doc-1' });
 		as.handleTextChange('block-1', 'text');
-		as.flushViaBeacon();
+		as.flushOnUnload();
 
 		expect(sendBeaconSpy).not.toHaveBeenCalled();
 	});
 
 	it('does nothing when there are no pending edits', () => {
 		const as = createBlockAutoSave({ saveFn: mockSaveFn, documentId: 'doc-1' });
-		as.flushViaBeacon();
+		as.flushOnUnload();
 
 		expect(mockFetch).not.toHaveBeenCalled();
 	});
@@ -143,9 +143,21 @@ describe('flushViaBeacon', () => {
 	it('cancels the debounce timer so saveFn is not also called', async () => {
 		const as = createBlockAutoSave({ saveFn: mockSaveFn, documentId: 'doc-1' });
 		as.handleTextChange('block-1', 'text');
-		as.flushViaBeacon();
+		as.flushOnUnload();
 
 		await vi.advanceTimersByTimeAsync(2000);
 		expect(mockSaveFn).not.toHaveBeenCalled();
 	});
+
+	it('does not send fetch if debounce already fired and pendingTexts is empty', async () => {
+		const as = createBlockAutoSave({ saveFn: mockSaveFn, documentId: 'doc-1' });
+		as.handleTextChange('block-1', 'text');
+		await vi.advanceTimersByTimeAsync(1500);
+		// debounce has fired; pendingTexts should be empty now
+		mockFetch.mockClear();
+
+		as.flushOnUnload();
+
+		expect(mockFetch).not.toHaveBeenCalled();
+	});
 });

frontend/src/lib/hooks/useBlockAutoSave.svelte.ts

@@ -94,10 +94,10 @@ export function createBlockAutoSave({ saveFn, documentId }: Options) {
 		saveStates.delete(blockId);
 	}
 
-	function flushViaBeacon(): void {
+	function flushOnUnload(): void {
 		for (const [blockId, text] of pendingTexts) {
 			clearDebounce(blockId);
-			fetch(`/api/documents/${documentId}/transcription-blocks/${blockId}`, {
+			void fetch(`/api/documents/${documentId}/transcription-blocks/${blockId}`, {
 				method: 'PUT',
 				headers: { 'Content-Type': 'application/json' },
 				body: JSON.stringify({ text }),
@@ -124,7 +124,7 @@ export function createBlockAutoSave({ saveFn, documentId }: Options) {
 		handleBlur,
 		handleRetry,
 		clearBlock,
-		flushViaBeacon,
+		flushOnUnload,
 		destroy
 	};
 }