Add gateway auto-detection and rename to Local CPE

Local CPE (192.168.100.1) is always monitored. On startup, probe
192.168.1.1, 192.168.0.1, 192.168.8.1, and 10.0.0.1 in parallel
and add whichever responds first as "Local Gateway".

README.md

@@ -53,8 +53,14 @@ code lives in `src/main.js` with a class-based architecture:
 
 - **11 WAN hosts**: datavi.be, Anthropic API, OpenAI API, AWS, GCP, Azure,
   DigitalOcean, Cloudflare, Fastly, Akamai, GitHub
-- **1 Local host**: Local Gateway (192.168.100.1), tracked separately from WAN
-  stats
+- **Local CPE**: Cable modem at 192.168.100.1 (always monitored)
+- **Local Gateway**: Auto-detected on startup by probing common default gateway
+  addresses (192.168.1.1, 192.168.0.1, 192.168.8.1, 10.0.0.1); first responder
+  wins. Note: modern browsers enforce Private Network Access restrictions that
+  block public-origin pages from reaching RFC1918 addresses, so local targets
+  only work when NetWatch is served from localhost or a private address.
+
+Local hosts are tracked separately from WAN stats.
 
 ### Latency measurement