feat: add email notifications when a note is shared

Send an email to collaborators when they are added to a shared note.
Uses nodemailer with optional SMTP configuration (auto-enabled when
SMTP_HOST is set). Users can opt out via the notifyOnShare preference.

https://claude.ai/code/session_01P4HSMr7QvKw9TMNikUJD9e

docker-compose.yml

@@ -11,6 +11,11 @@ services:
       - DATA_DIR=/data
       - DATABASE_URL=/data/crumbs.db
       - ORIGIN=${ORIGIN:-http://localhost:3000}
+      - SMTP_HOST=${SMTP_HOST:-}
+      - SMTP_PORT=${SMTP_PORT:-587}
+      - SMTP_USER=${SMTP_USER:-}
+      - SMTP_PASS=${SMTP_PASS:-}
+      - SMTP_FROM=${SMTP_FROM:-}
     restart: unless-stopped
 
 volumes: