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/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
 	};
 }