docs: add security setup section to README — passwd, root password, SSH check

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

README.md

@@ -2,6 +2,40 @@
 
 A monorepo setup to manage both Backend and Frontend of the Dental Manager application.
 
+## 🔒 Security Setup (First Thing on a New PC)
+
+### Step 1 — Change the user password
+
+```bash
+passwd
+```
+
+It will ask for the current password, then the new password twice.
+
+### Step 2 — Set the root password
+
+```bash
+sudo passwd root
+```
+
+Enter a strong password. This prevents anyone from getting root access with a blank password.
+
+### Step 3 — Check that SSH is not running
+
+```bash
+systemctl status sshd
+```
+
+If it shows "inactive (dead)" or "not found", you're safe — no one can remotely access this PC via SSH.
+
+If it shows "active (running)", disable it:
+
+```bash
+sudo systemctl stop sshd && sudo systemctl disable sshd
+```
+
+---
+
 ## 🖥️ Setup Guide (Fresh Machine)
 
 Follow these steps in order after cloning the repository.