fix(transcription): use navigator.sendBeacon for beforeunload save

Replace async executeSave in beforeunload handler with
navigator.sendBeacon — synchronous and reliable for page unload.
Sends pending text as JSON blob to the block update endpoint.

Fixes @Sara: "beforeunload handlers cannot reliably await async"

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

frontend/src/lib/components/TranscriptionEditView.svelte

@@ -7,13 +7,14 @@ import type { TranscriptionBlockData } from '$lib/types';
 type SaveState = 'idle' | 'saving' | 'saved' | 'fading' | 'error';
 
 type Props = {
+	documentId: string;
 	blocks: TranscriptionBlockData[];
 	onBlockFocus: (blockId: string) => void;
 	onSaveBlock: (blockId: string, text: string) => Promise<void>;
 	onDeleteBlock: (blockId: string) => Promise<void>;
 };
 
-let { blocks, onBlockFocus, onSaveBlock, onDeleteBlock }: Props = $props();
+let { documentId, blocks, onBlockFocus, onSaveBlock, onDeleteBlock }: Props = $props();
 
 let activeBlockId: string | null = $state(null);
 let saveStates = new SvelteMap<string, SaveState>();
@@ -115,9 +116,19 @@ function handleDelete(blockId: string) {
 	onDeleteBlock(blockId);
 }
 
+function flushViaBeacon() {
+	for (const [blockId, text] of pendingTexts) {
+		clearDebounce(blockId);
+		const url = `/api/documents/${documentId}/transcription-blocks/${blockId}`;
+		const body = JSON.stringify({ text });
+		navigator.sendBeacon(url, new Blob([body], { type: 'application/json' }));
+		pendingTexts.delete(blockId);
+	}
+}
+
 $effect(() => {
 	function onBeforeUnload() {
-		flushAllPending();
+		flushViaBeacon();
 	}
 
 	window.addEventListener('beforeunload', onBeforeUnload);

frontend/src/routes/documents/[id]/+page.svelte

@@ -199,6 +199,7 @@ onMount(() => {
 		{#if transcribeMode}
 			<div class="w-[400px] shrink-0 border-l border-line lg:w-[480px]">
 				<TranscriptionEditView
+					documentId={doc.id}
 					blocks={transcriptionBlocks}
 					onBlockFocus={handleBlockFocus}
 					onSaveBlock={saveBlock}