Add the Config view: edit and save the settings, applied without a restart

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/arr/update_test.go

@@ -46,3 +46,39 @@ func TestSetSeasonMonitoredUnknownSeason(t *testing.T) {
 		t.Error("want error")
 	}
 }
+
+func TestReconfigureSwitchesInstanceAndKey(t *testing.T) {
+	var gotKey, gotHost string
+	mk := func(host string) *httptest.Server {
+		return httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			gotKey, gotHost = r.Header.Get("X-Api-Key"), host
+			io.WriteString(w, `{"version":"1"}`)
+		}))
+	}
+	a, b := mk("a"), mk("b")
+	defer a.Close()
+	defer b.Close()
+
+	c := NewClient(a.URL, "key-a")
+	var out struct{ Version string }
+	if err := c.Do(context.Background(), http.MethodGet, "system/status", nil, nil, &out); err != nil || gotHost != "a" || gotKey != "key-a" {
+		t.Fatalf("before: host=%q key=%q err=%v", gotHost, gotKey, err)
+	}
+	c.Reconfigure(b.URL+"/", "key-b") // a trailing slash is trimmed like in NewClient
+	if err := c.Do(context.Background(), http.MethodGet, "system/status", nil, nil, &out); err != nil || gotHost != "b" || gotKey != "key-b" {
+		t.Fatalf("after: host=%q key=%q err=%v", gotHost, gotKey, err)
+	}
+	// Used from many goroutines while being changed (run with -race to see it matter).
+	done := make(chan bool)
+	go func() {
+		for i := 0; i < 50; i++ {
+			c.Reconfigure(a.URL, "k")
+			c.Reconfigure(b.URL, "k")
+		}
+		done <- true
+	}()
+	for i := 0; i < 50; i++ {
+		_ = c.Do(context.Background(), http.MethodGet, "system/status", nil, nil, &out)
+	}
+	<-done
+}