feat(ocr): allow re-running OCR when transcription blocks already exist

Add a collapsible OCR trigger below the block list in edit mode.
Uses a <details> element so it's unobtrusive — the primary workflow
is editing existing blocks, but users can expand to re-run OCR with
a confirmation dialog that warns about replacing existing blocks.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/messages/de.json

@@ -515,6 +515,7 @@
 	"ocr_confirm_title": "Vorhandene Transkription ersetzen?",
 	"ocr_confirm_body": "Alle {count} vorhandenen Blöcke werden gelöscht und durch die OCR-Ergebnisse ersetzt. Diese Aktion kann nicht rückgängig gemacht werden.",
 	"ocr_confirm_btn": "Ersetzen",
+	"ocr_rerun_label": "OCR erneut ausführen…",
 	"ocr_progress_heading": "OCR läuft",
 	"ocr_progress_page": "Seite {current} von {total}",
 	"ocr_error_heading": "OCR fehlgeschlagen",

frontend/messages/en.json

@@ -515,6 +515,7 @@
 	"ocr_confirm_title": "Replace existing transcription?",
 	"ocr_confirm_body": "All {count} existing blocks will be deleted and replaced with OCR results. This action cannot be undone.",
 	"ocr_confirm_btn": "Replace",
+	"ocr_rerun_label": "Re-run OCR…",
 	"ocr_progress_heading": "OCR running",
 	"ocr_progress_page": "Page {current} of {total}",
 	"ocr_error_heading": "OCR failed",

frontend/messages/es.json

@@ -515,6 +515,7 @@
 	"ocr_confirm_title": "¿Reemplazar transcripción existente?",
 	"ocr_confirm_body": "Los {count} bloques existentes serán eliminados y reemplazados con los resultados del OCR. Esta acción no se puede deshacer.",
 	"ocr_confirm_btn": "Reemplazar",
+	"ocr_rerun_label": "Ejecutar OCR de nuevo…",
 	"ocr_progress_heading": "OCR en curso",
 	"ocr_progress_page": "Página {current} de {total}",
 	"ocr_error_heading": "OCR fallido",