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/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"