Restructure ESP32 firmware into modules, fix bugs, add tests

Structure (src/esp32/):
- logic/: DisplayController, schedule, power timer, LED pattern; pure C++
- display/: FlipdotFrames and bitmaps
- nano_link/: NanoLink (relay, serial, READY, bootloader) and flash bridge
- net/: WiFi/OTA/panic window, Home Assistant
- config.h with all settings; libs.h (globals in a header) removed

Bug fixes:
- Home Assistant switches now report their state back and start as ON
- loop() no longer blocks: WiFi.waitForConnectResult() (up to 60 s
  without WiFi) and getLocalTime() (5 s before NTP sync) were called
  every iteration
- Power switch off now powers off after 1 s even when a 30 s countdown
  is already running
- millis() overflow safe power-off timer; no frame catch-up bursts
- No garbage time string before NTP sync; 'Connecting...' stays instead
- Frames are only sent when the screen changes instead of every second
- Reset reasons are logged once instead of every 100 ms
- Removed dead code (unused fonts, WiFiMulti, rotated bitmap, ...)

Tests: new test_esp32 environment (26 tests); native renamed to test_nano

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

src/esp32/net/home_assistant.cpp

@@ -0,0 +1,55 @@
+#include "net/home_assistant.h"
+
+#include <ArduinoHA.h>
+#include <WiFi.h>
+
+#include "config.h"
+#include "credentials.h"
+
+static WiFiClient client;
+static HADevice device(HA_DEVICE_ID, sizeof(HA_DEVICE_ID));
+static HAMqtt mqtt(client, device);
+
+// Unique ids: do not change, Home Assistant's entities are linked to them
+static HASwitch clockSwitch("FlipDotDisplayClock");
+static HASwitch powerSwitch("FlipDotDisplayPower");
+
+static DisplayController *displayController = nullptr;
+
+// The switches are not optimistic: the new state must be reported back to Home Assistant
+static void onClockCommand(bool state, HASwitch *sender)
+{
+	displayController->setClockEnabled(state);
+	sender->setState(state);
+}
+
+static void onPowerCommand(bool state, HASwitch *sender)
+{
+	displayController->setPowerEnabled(state);
+	sender->setState(state);
+}
+
+void homeAssistantBegin(DisplayController &controller)
+{
+	displayController = &controller;
+
+	device.setName(HA_DEVICE_NAME);
+	device.setSoftwareVersion(HA_SOFTWARE_VERSION);
+
+	clockSwitch.setName("FlipDot Display Clock");
+	clockSwitch.setIcon("mdi:lightbulb");
+	clockSwitch.setCurrentState(controller.clockEnabled());
+	clockSwitch.onCommand(onClockCommand);
+
+	powerSwitch.setName("FlipDot Display Power");
+	powerSwitch.setIcon("mdi:lightbulb");
+	powerSwitch.setCurrentState(controller.powerEnabled());
+	powerSwitch.onCommand(onPowerCommand);
+
+	mqtt.begin(BROKER_ADDR, MQTT_USER, MQTT_PASSWORD);
+}
+
+void homeAssistantLoop()
+{
+	mqtt.loop();
+}