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/Cpp20/smoke_test.cpp

@@ -0,0 +1,15 @@
+#include <ArduinoJson.h>
+
+#include <catch.hpp>
+#include <string>
+
+TEST_CASE("C++20 smoke test") {
+  StaticJsonDocument<128> doc;
+
+  deserializeJson(doc, "{\"hello\":\"world\"}");
+  REQUIRE(doc["hello"] == "world");
+
+  std::string json;
+  serializeJson(doc, json);
+  REQUIRE(json == "{\"hello\":\"world\"}");
+}