fix(sensor-proxy): add --proxy-url flag for manual URL override

Closes #826

The error messages suggested using --proxy-url but the flag was never
implemented. This adds the flag so users can manually specify the
proxy URL when:
- Auto IP detection produces malformed results
- The desired IP is not the primary IP
- Multi-homed hosts need a specific interface

scripts/install-sensor-proxy.sh

@@ -1163,6 +1163,7 @@ UNINSTALL=false
 PURGE=false
 CONTROL_PLANE_TOKEN=""
 CONTROL_PLANE_REFRESH=""
+PROXY_URL=""  # Manual override for advertised proxy URL
 SHORT_HOSTNAME=$(hostname -s 2>/dev/null || hostname | cut -d'.' -f1)
 
 while [[ $# -gt 0 ]]; do
@@ -1215,6 +1216,10 @@ while [[ $# -gt 0 ]]; do
             PURGE=true
             shift
             ;;
+        --proxy-url)
+            PROXY_URL="$2"
+            shift 2
+            ;;
         *)
             print_error "Unknown option: $1"
             exit 1
@@ -1964,7 +1969,12 @@ if [[ "$HTTP_MODE" == true ]]; then
     # Setup TLS certificates
     setup_tls_certificates "" ""  # Empty params = auto-generate
 
-    # Determine proxy URL - use IP address for reliable network access
+    # Determine proxy URL - use provided URL or auto-detect IP address
+    if [[ -n "$PROXY_URL" ]]; then
+        # User provided --proxy-url, use it directly
+        print_info "Using manually specified proxy URL: $PROXY_URL"
+    else
+        # Auto-detect from primary IP
         PRIMARY_IP=$(hostname -I 2>/dev/null | awk '{print $1}')
         if [[ -z "$PRIMARY_IP" ]]; then
             print_error "Failed to determine primary IP address"
@@ -1995,6 +2005,7 @@ if [[ "$HTTP_MODE" == true ]]; then
 
         PROXY_URL="https://${PRIMARY_IP}:${PORT_NUMBER}"
         print_info "Proxy will be accessible at: $PROXY_URL"
+    fi
 
     # Register with Pulse and get auth/control tokens
     registration_response=$(register_with_pulse "$PULSE_SERVER" "$SHORT_HOSTNAME" "$PROXY_URL" "http")