feat(training): add Paraglide i18n to training UI components and wire SegmentationTrainingCard

- Convert TrainingHistory, OcrTrainingCard, SegmentationTrainingCard, and
  TranscriptionBlock "Nur Segmentierung" badge to use Paraglide message keys
- Add availableSegBlocks to TrainingInfoResponse to expose segmentation
  block count in the training info endpoint
- Wire SegmentationTrainingCard into admin/system page below OCR training card
- Update api.ts with availableSegBlocks field

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

frontend/src/lib/components/TranscriptionBlock.svelte

@@ -27,6 +27,7 @@ type Props = {
 	onMoveDown?: () => void;
 	isFirst?: boolean;
 	isLast?: boolean;
+	source?: 'MANUAL' | 'OCR';
 };
 
 let {
@@ -48,7 +49,8 @@ let {
 	onMoveUp,
 	onMoveDown,
 	isFirst = false,
-	isLast = false
+	isLast = false,
+	source = 'MANUAL'
 }: Props = $props();
 
 let localText = $state(text);
@@ -172,6 +174,11 @@ function handleTextareaMouseUp() {
 					{label}
 				</span>
 			{/if}
+			{#if (!text || text.trim() === '') && source === 'MANUAL'}
+				<span class="rounded bg-muted px-1.5 py-0.5 text-xs font-medium text-ink-3"
+					>{m.transcription_block_segmentation_only()}</span
+				>
+			{/if}
 		</div>
 
 		<!-- Textarea -->