feat(transcription): add dashed next-block CTA below block list

Shows a muted dashed-outline box after the last block:
"Markiere eine weitere Passage im Scan, um Block N anzulegen"
Guides new users on how to create additional blocks.

Matches the spec's empty block CTA design (S1, bottom of block list).
i18n key transcription_next_block_cta added for de/en/es.

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

frontend/messages/de.json

@@ -453,6 +453,7 @@
 	"transcription_block_delete_confirm": "Block und alle zugehörigen Kommentare wirklich löschen?",
 	"transcription_block_history_btn": "Verlauf",
 	"transcription_empty_cta": "Markiere einen Bereich auf dem Scan, um mit der Transkription zu beginnen",
+	"transcription_next_block_cta": "Markiere eine weitere Passage im Scan, um Block {number} anzulegen",
 	"transcription_draw_tooltip": "Klicken und ziehen, um einen Textbereich zu markieren",
 	"transcription_quote_stale": "Zitat aus älterer Version",
 	"transcription_block_conflict": "Dieser Block wurde von jemand anderem geändert — bitte neu laden"

frontend/messages/en.json

@@ -453,6 +453,7 @@
 	"transcription_block_delete_confirm": "Really delete this block and all its comments?",
 	"transcription_block_history_btn": "History",
 	"transcription_empty_cta": "Mark a region on the scan to start transcribing",
+	"transcription_next_block_cta": "Mark another passage on the scan to create block {number}",
 	"transcription_draw_tooltip": "Click and drag to mark a text region",
 	"transcription_quote_stale": "Quote from an older version",
 	"transcription_block_conflict": "This block was changed by someone else — please reload"

frontend/messages/es.json

@@ -453,6 +453,7 @@
 	"transcription_block_delete_confirm": "¿Realmente eliminar este bloque y todos sus comentarios?",
 	"transcription_block_history_btn": "Historial",
 	"transcription_empty_cta": "Marque una región en el escaneo para comenzar a transcribir",
+	"transcription_next_block_cta": "Marque otro pasaje en el escaneo para crear el bloque {number}",
 	"transcription_draw_tooltip": "Haga clic y arrastre para marcar una región de texto",
 	"transcription_quote_stale": "Cita de una versión anterior",
 	"transcription_block_conflict": "Este bloque fue cambiado por otra persona — por favor recargue"

frontend/src/lib/components/TranscriptionEditView.svelte

@@ -173,6 +173,13 @@ $effect(() => {
 					/>
 				</div>
 			{/each}
+
+			<!-- Next block CTA — dashed outline hint -->
+			<div
+				class="flex items-center justify-center rounded border border-dashed border-line px-4 py-5 text-center font-sans text-sm text-ink-3"
+			>
+				{m.transcription_next_block_cta({ number: sortedBlocks.length + 1 })}
+			</div>
 		</div>
 	{:else}
 		<div class="flex flex-1 flex-col items-center justify-center px-6 py-12 text-center">