Implement configurable STATE_DIR for state files

Move processed_message_ids.txt and last_run.txt to STATE_DIR,
configurable via .env. Install script creates ./state by default.

Changes:
- Add STATE_DIR to .env.example and config loading
- Update EmailForwarder to use os.path.join for file paths
- Add os.makedirs for state dir creation
- Update README with STATE_DIR option

.env.example

@@ -7,6 +7,7 @@ DEST_PORT=993
 DEST_USER=your@mxroute.com
 DEST_PASS=password
 FOLDERS=INBOX,Sent
+STATE_DIR=./state
 DRY_RUN=false
 UPTIME_SUCCESS_URL=https://your-uptime-instance.com/api/push/your-token?status=up&msg=OK&ping=
 UPTIME_FAIL_URL=https://your-uptime-instance.com/api/push/your-token?status=down&msg=Failed