feat(person-mention): update transcription placeholder with @mention discoverability hint

Replaces the generic "Type text here..." placeholder in TranscriptionBlock
with copy that teaches the @Name trigger inline (Leonie Voss design review,
issue #370). No new DOM, no new i18n keys — just the three existing
`transcription_block_placeholder` strings.

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

frontend/messages/de.json

@@ -502,7 +502,7 @@
 	"doc_details_more_receivers": "+{count} weitere",
 	"transcription_mode_label": "Transkribieren",
 	"transcription_mode_stop": "Fertig",
-	"transcription_block_placeholder": "Text hier eingeben...",
+	"transcription_block_placeholder": "Text eingeben — mit @Name eine Person aus dem Archiv verknüpfen",
 	"transcription_block_save_saving": "Speichere...",
 	"transcription_block_save_saved": "Gespeichert",
 	"transcription_block_save_error": "Nicht gespeichert",

frontend/messages/en.json

@@ -502,7 +502,7 @@
 	"doc_details_more_receivers": "+{count} more",
 	"transcription_mode_label": "Transcribe",
 	"transcription_mode_stop": "Done",
-	"transcription_block_placeholder": "Type text here...",
+	"transcription_block_placeholder": "Type text — use @name to link a person from the archive",
 	"transcription_block_save_saving": "Saving...",
 	"transcription_block_save_saved": "Saved",
 	"transcription_block_save_error": "Not saved",

frontend/messages/es.json

@@ -502,7 +502,7 @@
 	"doc_details_more_receivers": "+{count} más",
 	"transcription_mode_label": "Transcribir",
 	"transcription_mode_stop": "Listo",
-	"transcription_block_placeholder": "Escriba el texto aquí...",
+	"transcription_block_placeholder": "Escriba el texto — use @nombre para vincular a una persona del archivo",
 	"transcription_block_save_saving": "Guardando...",
 	"transcription_block_save_saved": "Guardado",
 	"transcription_block_save_error": "No guardado",