fix DNS failure on network change with upstream re-detection

Upstream DNS was resolved once at startup and never updated. Switching
Wi-Fi networks made all queries fail until restart.

Now spawns a background task (every 30s) that re-runs system DNS
discovery and swaps the upstream atomically if it changed. Also flushes
stale LAN peers from the old network on change.

Only activates when upstream is auto-detected (not explicitly configured).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/lan.rs

@@ -57,6 +57,10 @@ impl PeerStore {
             })
             .collect()
     }
+
+    pub fn clear(&mut self) {
+        self.peers.clear();
+    }
 }
 
 // --- Multicast ---