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/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}