CRITICAL FIX: Add safety checks to prevent breaking system files

ISSUE: Previous version could break sudo and system files
The recursive chown/chmod commands in configure_user_shares() and
create_share() could modify system directories like /usr, /etc, /home
causing critical system breakage including sudo permissions.

ROOT CAUSE:
- No validation of paths before recursive operations
- Could modify /, /usr, /home and other system directories
- Broke /usr/bin/sudo permissions (needs uid 0 and setuid bit)

SOLUTION: Added comprehensive path safety checks

New function: is_safe_path_for_permissions()
- Blacklists ALL dangerous system paths: /, /usr, /etc, /bin, /var, etc.
- Only allows /mnt/* (external disk mounts)
- Only allows /home/user/subdir (not /home or /home/user itself)
- Returns error for any system directory

Protection applied to:
1. create_share() - disk share creation (line 326)
2. configure_user_shares() - user access configuration (line 869)

Behavior:
- Safe paths (/mnt/*): Permissions applied normally
- Unsafe paths: Prints warning, skips permission changes
- Users must manually set permissions for system directories

Emergency fix instructions added to README:
- How to fix broken sudo (chown root:root /usr/bin/sudo && chmod 4755)
- Multiple recovery methods (root shell, su, recovery mode)
- Clear warning about older versions

This prevents catastrophic system breakage while still allowing
proper multi-user access for external disk shares.

APOLOGIES TO USERS: If you were affected by the previous version,
I'm deeply sorry for breaking your system. Please follow the
recovery instructions in the README.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

README.md

@@ -200,6 +200,36 @@ Perfect for initial setup or adding multiple disks at once!
 
 ## Troubleshooting
 
+### CRITICAL: If sudo is broken after running user-access
+
+**Symptoms**: `sudo: /usr/bin/sudo must be owned by uid 0 and have the setuid bit set`
+
+**This happened if you ran an older version (before v1.3) that modified system directories.**
+
+**Fix (choose one method):**
+
+**Method 1 - If you're still root in a shell:**
+```bash
+chown root:root /usr/bin/sudo
+chmod 4755 /usr/bin/sudo
+```
+
+**Method 2 - Switch to root user:**
+```bash
+su -
+chown root:root /usr/bin/sudo
+chmod 4755 /usr/bin/sudo
+exit
+```
+
+**Method 3 - Recovery mode:**
+1. Reboot and select recovery/single-user mode in GRUB
+2. Mount filesystem: `mount -o remount,rw /`
+3. Fix sudo: `chown root:root /usr/bin/sudo && chmod 4755 /usr/bin/sudo`
+4. Reboot normally
+
+**After fixing sudo, update to the latest version of FSA which has safety checks!**
+
 ### Check service status
 ```bash
 sudo systemctl status smbd nmbd