feat(ui): add collapsible PDF strip and abbreviated labels on mobile

PDF viewer collapses to 70px on mobile in read mode, expandable to
50vh. Toggle button with chevron. Paragraph tap auto-expands strip.
Mode toggle abbreviates to "Bearb." on small screens.

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

frontend/src/lib/components/TranscriptionPanelHeader.svelte

@@ -54,12 +54,13 @@ function handleReadClick() {
 				? 'bg-primary text-primary-fg'
 				: 'text-ink-2 hover:text-ink'}"
 		>
-			{m.mode_edit()}
+			<span class="md:hidden">{m.mode_edit_short()}</span>
+			<span class="hidden md:inline">{m.mode_edit()}</span>
 		</button>
 	</div>
 
-	<!-- Status line -->
-	<p class="text-xs text-ink-2">
+	<!-- Status line (hidden on mobile to save space) -->
+	<p class="hidden text-xs text-ink-2 md:block">
 		{#if blockCount === 1}
 			{m.transcription_status_section()}
 		{:else}