Import ESP_UniClock (upstream 1189810)

Vendored from https://github.com/gpeter62/ESP_UniClock: the sketch
source, the web UI and the libraries the ESP8266 build needs, unpacked
from the bundled zips. Adds a PlatformIO project for the ESP-12. The
source is unchanged; local changes follow as separate commits.

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

firmware/uniclock/lib/ArduinoJson/extras/tests/JsonVariant/add.cpp

@@ -0,0 +1,46 @@
+// ArduinoJson - https://arduinojson.org
+// Copyright © 2014-2022, Benoit BLANCHON
+// MIT License
+
+#include <ArduinoJson.h>
+#include <stdint.h>
+#include <catch.hpp>
+
+TEST_CASE("JsonVariant::add()") {
+  DynamicJsonDocument doc(4096);
+  JsonVariant var = doc.to<JsonVariant>();
+
+  SECTION("add integer to new variant") {
+    var.add(42);
+
+    REQUIRE(var.as<std::string>() == "[42]");
+  }
+
+  SECTION("add const char* to new variant") {
+    var.add("hello");
+
+    REQUIRE(var.as<std::string>() == "[\"hello\"]");
+  }
+
+  SECTION("add std::string to new variant") {
+    var.add(std::string("hello"));
+
+    REQUIRE(var.as<std::string>() == "[\"hello\"]");
+  }
+
+  SECTION("add integer to integer") {
+    var.set(123);
+
+    var.add(456);  // no-op
+
+    REQUIRE(var.as<std::string>() == "123");
+  }
+
+  SECTION("add integer to object") {
+    var["val"] = 123;
+
+    var.add(456);  // no-op
+
+    REQUIRE(var.as<std::string>() == "{\"val\":123}");
+  }
+}