show upstream DNS in stats API and dashboard footer

Expose current upstream address in /stats response. Dashboard footer
now shows "Upstream: x.x.x.x:53" — updates live when the network
watcher swaps the upstream.

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

src/api.rs

@@ -126,6 +126,7 @@ struct QueryLogResponse {
 #[derive(Serialize)]
 struct StatsResponse {
     uptime_secs: u64,
+    upstream: String,
     queries: QueriesStats,
     cache: CacheStats,
     overrides: OverrideStats,
@@ -435,8 +436,11 @@ async fn stats(State(ctx): State<Arc<ServerCtx>>) -> Json<StatsResponse> {
     let override_count = ctx.overrides.lock().unwrap().active_count();
     let bl_stats = ctx.blocklist.lock().unwrap().stats();
 
+    let upstream = ctx.upstream.lock().unwrap().to_string();
+
     Json(StatsResponse {
         uptime_secs: snap.uptime_secs,
+        upstream,
         queries: QueriesStats {
             total: snap.total,
             forwarded: snap.forwarded,