i18n(briefwechsel): ThumbnailRow direction label via Paraglide

Adds row_direction_sent / row_direction_received keys across the
three locale files (de: Gesendet/Empfangen, en: Sent/Received, es:
Enviada/Recibida) and routes ThumbnailRow's directionLabel through
Paraglide. An English or Spanish screen-reader user now hears
"Sent:" / "Enviada:" in their language, matching the DistributionBar
i18n pass.

Refs #305
Fixes @leonievoss round-2 follow-up from PR review

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

frontend/messages/de.json

@@ -167,6 +167,8 @@
 	"conv_no_party": "—",
 	"dist_bar_segment": "{count} von {name}",
 	"dist_bar_aria": "Briefverteilung in diesem Zeitraum: {outCount} von {senderName}, {inCount} von {receiverName}",
+	"row_direction_sent": "Gesendet",
+	"row_direction_received": "Empfangen",
 	"admin_heading": "Admin Dashboard",
 	"admin_tab_users": "Benutzer",
 	"admin_tab_groups": "Gruppen",

frontend/messages/en.json

@@ -167,6 +167,8 @@
 	"conv_no_party": "—",
 	"dist_bar_segment": "{count} from {name}",
 	"dist_bar_aria": "Letter distribution in this period: {outCount} from {senderName}, {inCount} from {receiverName}",
+	"row_direction_sent": "Sent",
+	"row_direction_received": "Received",
 	"admin_heading": "Admin Dashboard",
 	"admin_tab_users": "Users",
 	"admin_tab_groups": "Groups",

frontend/messages/es.json

@@ -167,6 +167,8 @@
 	"conv_no_party": "—",
 	"dist_bar_segment": "{count} de {name}",
 	"dist_bar_aria": "Distribución de cartas en este período: {outCount} de {senderName}, {inCount} de {receiverName}",
+	"row_direction_sent": "Enviada",
+	"row_direction_received": "Recibida",
 	"admin_heading": "Panel de administración",
 	"admin_tab_users": "Usuarios",
 	"admin_tab_groups": "Grupos",