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 () => {