CRITICAL: Fix domain detection bug in get_cpanel_user_domains

Root cause: grep -F with regex anchor
- grep -F means 'fixed string' (no regex)
- Pattern 'grep -F "$username\$"' was looking for literal backslash-dollar
- Changed to 'grep "${username}$"' (regex mode with end-of-line anchor)

Impact:
- PHP optimizer showed 0 domains analyzed
- Server memory check showed 0MB required
- ALL domain-based functionality was broken

This is why the script appeared to work but returned no data.

Files fixed:
- lib/user-manager.sh:254,258 (2 lines changed)

lib/user-manager.sh

@@ -251,11 +251,11 @@ get_cpanel_user_domains() {
     local username="$1"
 
     # Primary domain (format: domain: user)
-    grep -F ": ${username}" /etc/trueuserdomains 2>/dev/null | grep -F "$username\$" 2>/dev/null | cut -d: -f1 || true
+    grep -F ": ${username}" /etc/trueuserdomains 2>/dev/null | grep "${username}$" 2>/dev/null | cut -d: -f1 || true
 
     # Addon domains
     if [ -f "/etc/userdatadomains" ]; then
-        grep -F "==${username}" /etc/userdatadomains 2>/dev/null | grep -F "$username\$" 2>/dev/null | cut -d: -f1 || true
+        grep -F "==${username}" /etc/userdatadomains 2>/dev/null | grep "${username}$" 2>/dev/null | cut -d: -f1 || true
     fi
 }