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/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">