polish(transcribe): review nits — kbd size, focus ring, guard, action doc (#327)

Review follow-up (Leonie, Felix, Markus): bump cheatsheet key caps to text-sm
for the 60+ audience, add a focus-visible ring to the close button, simplify
the draw-hint guard to {#if drawArmed} (the $effect already clears it outside
edit mode), and document why the transcribeShortcuts action ignores its node
and binds to window.

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

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

@@ -422,7 +422,7 @@ onMount(() => {
 		{/if}
 	</div>
 
-	{#if drawArmed && panelMode === 'edit'}
+	{#if drawArmed}
 		<div
 			class="pointer-events-none absolute bottom-4 left-1/2 z-50 -translate-x-1/2 rounded-full bg-ink px-4 py-2 font-sans text-xs text-white shadow-lg"
 			role="status"