feat(mail): sender display name (SMTP_FROM_NAME, default MindSift)

A bare SMTP_FROM address gets the display name in the From header; a name
already in SMTP_FROM wins, the envelope keeps the bare address. Also a
field in the admin mail settings.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docker-compose.yml

@@ -29,6 +29,7 @@ services:
       - SMTP_USER=${SMTP_USER:-}
       - SMTP_PASS=${SMTP_PASS:-}
       - SMTP_FROM=${SMTP_FROM:-}
+      - SMTP_FROM_NAME=${SMTP_FROM_NAME:-MindSift}
       - SMTP_TLS=${SMTP_TLS:-true}
       - SMTP_SKIP_VERIFY=${SMTP_SKIP_VERIFY:-false}
       # Impressum / Datenschutz