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/nano_link/nano_link.h

@@ -0,0 +1,36 @@
+#pragma once
+
+#include <Arduino.h>
+
+#include "logic/display_controller.h"
+
+// The relay that powers the panels and the Nano, and the serial link to the Nano.
+class NanoLink : public DisplayPower
+{
+public:
+	NanoLink(HardwareSerial &serial, int rx_pin, int tx_pin, int relay_pin, int reset_pin);
+
+	// Set up the pins; the display starts switched off
+	void begin();
+
+	// The serial port to the Nano (for the BROSE9323 driver)
+	Stream &stream();
+
+	bool isOn() override;
+	// Switch on, then wait for the Nano's READY (or NANO_BOOT_TIMEOUT)
+	void on() override;
+	// Switch off and release the TX pin
+	void off() override;
+
+	// For flashing: restart the Nano so its bootloader runs, and talk to it at `baud`
+	void startBootloader(uint32_t baud);
+
+private:
+	HardwareSerial &_serial;
+	const int _rx_pin, _tx_pin, _relay_pin, _reset_pin;
+	bool _on = false;
+
+	void _beginSerial(uint32_t baud);
+	void _drain();
+	void _waitForReady();
+};