i18n(transcription): add reader read-label and panel title strings (#697)

transcription_read_label ("Transkription lesen") for the read-only entry
control and transcription_panel_title ("Transkription") for the plain
header readers see instead of the Lesen/Bearbeiten toggle.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/document/DocumentMobileMenu.svelte

@@ -5,6 +5,7 @@ import { clickOutside } from '$lib/shared/actions/clickOutside';
 type Props = {
 	canWrite: boolean;
 	isPdf: boolean;
+	hasTranscription: boolean;
 	transcribeMode: boolean;
 	filePath?: string | null;
 	originalFilename?: string | null;
@@ -14,12 +15,18 @@ type Props = {
 let {
 	canWrite,
 	isPdf,
+	hasTranscription,
 	transcribeMode = $bindable(),
 	filePath = null,
 	originalFilename = null,
 	fileUrl
 }: Props = $props();
 
+const canOpenTranscription = $derived((canWrite || hasTranscription) && isPdf);
+const transcriptionLabel = $derived(
+	canWrite ? m.transcription_mode_label() : m.transcription_read_label()
+);
+
 let mobileMenuOpen = $state(false);
 
 function startTranscribe() {
@@ -50,10 +57,10 @@ function startTranscribe() {
 			role="menu"
 			class="absolute top-full right-0 z-50 mt-1 min-w-[200px] rounded-md border border-line bg-surface p-2 shadow-lg"
 		>
-			{#if canWrite && isPdf && !transcribeMode}
+			{#if canOpenTranscription && !transcribeMode}
 				<button
 					onclick={startTranscribe}
-					aria-label={m.transcription_mode_label()}
+					aria-label={transcriptionLabel}
 					aria-pressed={false}
 					class="flex w-full items-center gap-2 rounded px-3 py-2 text-left text-[16px] text-ink transition hover:bg-muted focus-visible:ring-2 focus-visible:ring-primary"
 				>
@@ -70,7 +77,7 @@ function startTranscribe() {
 							d="M19.5 14.25v-2.625a3.375 3.375 0 00-3.375-3.375h-1.5A1.125 1.125 0 0113.5 7.125v-1.5a3.375 3.375 0 00-3.375-3.375H8.25m0 12.75h7.5m-7.5 3H12M10.5 2.25H5.625c-.621 0-1.125.504-1.125 1.125v17.25c0 .621.504 1.125 1.125 1.125h12.75c.621 0 1.125-.504 1.125-1.125V11.25a9 9 0 00-9-9z"
 						/>
 					</svg>
-					{m.transcription_mode_label()}
+					{transcriptionLabel}
 				</button>
 			{/if}