a11y(transcription): remove redundant aria-live="polite" from alert div

role="alert" already implies aria-live="assertive". The polite override
caused screen readers to wait for the current announcement to finish
before reading the error — too gentle for a failure state the user just
triggered. Dropping the attribute restores the implicit assertive
behaviour.

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

frontend/src/lib/document/transcription/TranscriptionEditView.svelte

@@ -224,7 +224,6 @@ async function handleLabelToggle(label: string) {
 			{#if markAllError}
 				<div
 					role="alert"
-					aria-live="polite"
 					class="mt-1.5 flex items-center gap-2 rounded-sm border border-red-200 bg-red-50 px-3 py-2 font-sans text-sm text-red-700"
 				>
 					<span class="flex-1">{markAllError}</span>