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/api.rs

@@ -341,8 +341,9 @@ async fn diagnose(
     }
 
     // Check upstream (async, no locks held)
+    let upstream = *ctx.upstream.lock().unwrap();
     let (upstream_matched, upstream_detail) =
-        forward_query_for_diagnose(&domain_lower, ctx.upstream, ctx.timeout).await;
+        forward_query_for_diagnose(&domain_lower, upstream, ctx.timeout).await;
     steps.push(DiagnoseStep {
         source: "upstream".to_string(),
         matched: upstream_matched,